What Is Murder? Definition, Degrees, and Defenses

Murder is the unlawful killing of another person committed with “malice aforethought” — a legal term that covers intent to kill, intent to cause serious bodily harm, extreme recklessness toward human life, or a death that occurs during certain dangerous felonies.¹ Most states divide murder into degrees based on how much planning went into the act and how dangerous the underlying conduct was. The degree matters: it determines whether someone faces a set term of years, life in prison, or in some jurisdictions, the death penalty.

What Makes a Killing Murder

Every murder conviction rests on three things: a criminal act, a guilty mental state, and a causal link between them. The act is usually straightforward — someone caused another person’s death. The mental state is where most courtroom fighting happens.

Despite the archaic name, malice aforethought does not always require advance planning. Courts have historically recognized four ways to satisfy it:

Intent to kill. The defendant meant to cause the victim’s death.
Intent to cause serious bodily harm. The defendant meant to severely injure someone, and that person died. A murder conviction can follow even if the defendant never wanted anyone dead.²
Depraved heart. The defendant acted with extreme recklessness showing total disregard for human life, such as firing a gun into a crowd without aiming at anyone in particular.
Felony murder. Someone died during the commission of a dangerous felony, and the law treats the decision to commit that felony as a substitute for intent to kill.

Prosecutors must also show the defendant actually caused the death. That involves two tests. First, the “but for” test: would the victim have died if the defendant had done nothing? Second, the death must be a foreseeable result of the defendant’s conduct rather than a bizarre coincidence. Where something else contributes to the death after the initial act — negligent medical care following a stabbing, for example — the original attacker usually remains liable because such complications are foreseeable. Only a truly extraordinary intervening event breaks the chain, and courts take a broad view of what counts as foreseeable.

First-Degree Murder

First-degree murder sits at the top of the severity ladder. The defining feature is premeditation and deliberation: the killer thought about what they were going to do, made a conscious choice, and then followed through. How long that reflection lasted matters less than people assume. Courts have upheld first-degree convictions where the premeditation period lasted only seconds, as long as the evidence showed the defendant formed a clear intent before acting.

Deliberation means the decision was made with a calm, reasoning mind rather than in the grip of rage or panic. Prosecutors look for circumstantial evidence of planning: acquiring a weapon beforehand, surveilling the victim, luring them to a specific location, or taking steps to avoid detection afterward.

The federal murder statute also classifies certain methods as automatic first-degree murder regardless of how much time the killer spent planning. Killing by poison or by lying in wait both qualify, as does any killing committed during the course of specific dangerous felonies such as arson, robbery, kidnapping, burglary, espionage, or sexual abuse.¹ The logic is that choosing one of these inherently lethal methods, or committing murder alongside another violent crime, demonstrates the kind of calculated dangerousness first-degree murder is designed to punish.

Federal penalties for first-degree murder are death or life imprisonment.¹ Most states follow a similar pattern, with mandatory minimums that vary widely. Some start as low as 25 years; others mandate life without the possibility of parole. Capital punishment remains available for first-degree murder in some jurisdictions, though its use has declined significantly over the past two decades.

Second-Degree Murder

Second-degree murder covers intentional killings that happen without premeditation. The person meant to kill or cause serious harm but acted in the moment rather than carrying out a plan. A bar fight that escalates from fists to a fatal knife wound is the classic example. The killer intended the act but did not walk in with a strategy.

This category also captures depraved heart murder. Some behavior is so recklessly dangerous that the law treats it as equivalent to intentional killing even when the defendant did not specifically want anyone to die. Playing Russian roulette with someone, driving at extreme speed through a residential neighborhood, or shooting into an occupied building all qualify. The defendant may not have picked a target, but their conduct showed such complete indifference to whether people lived or died that the law holds them to the same standard as someone who acted on purpose.²

Under the federal statute, second-degree murder is defined simply as any other murder that does not meet the first-degree criteria. The penalty is imprisonment for any term of years or for life.¹ That wide sentencing range gives judges significant discretion to account for how close the case comes to first-degree conduct versus something closer to manslaughter.

Felony Murder

The felony murder rule is one of the most controversial doctrines in American criminal law. It allows a murder conviction even when the defendant never intended to kill anyone. The theory is that if you choose to commit a dangerous felony and someone dies as a result, the law transfers your intent to commit that felony onto the death itself.

Federal law lists the felonies that trigger this rule: arson, robbery, burglary, kidnapping, espionage, sabotage, sexual abuse, child abuse, and escape.¹ Most states maintain their own lists, and some cast a wider net than the federal statute. The common thread is that the underlying crime must be inherently dangerous to human life.

The reach of this rule can be startling. If two people commit an armed robbery and the store clerk dies of a heart attack during the holdup, both robbers face murder charges. If a getaway driver waits in the car while an accomplice accidentally kills someone inside, the driver faces murder charges too.

One important limit is the merger doctrine. If the underlying felony is essentially an assault — the same violent act that caused the death — courts in many states will not allow the prosecution to use the felony murder shortcut. Otherwise, every assault that killed someone would automatically become felony murder, and prosecutors could bypass the intent requirements for murder in virtually every case. A California court illustrated the point by blocking a felony murder charge where the underlying crime was a drive-by shooting, since the shooting was the same conduct that caused the death.

A handful of states have abolished the felony murder rule entirely, and several others have created affirmative defenses for participants who did not personally commit the killing, were not armed, and had no reason to believe anyone would be killed. Even so, the federal system and the large majority of states continue to apply the rule.

How Murder Differs From Manslaughter

The line between murder and manslaughter is malice. Murder requires it; manslaughter does not. Federal law defines manslaughter as the unlawful killing of a person without malice and divides it into two categories.³

Voluntary Manslaughter

Voluntary manslaughter is an intentional killing committed in the heat of passion after adequate provocation. The defendant meant to kill, but the circumstances reduce the moral blame. A person who walks in on a spouse in an act of infidelity and kills in a blind rage might face voluntary manslaughter rather than murder, not because the killing was acceptable but because the law recognizes that extreme provocation can overwhelm rational thought.

Two conditions apply. The provocation must be severe enough that a reasonable person could lose self-control; a minor insult does not qualify. And the killing must happen before the defendant has time to cool off. If the same person leaves, drives around for two hours, comes back, and then kills, that cooling-off period transforms the charge back into murder. The federal penalty for voluntary manslaughter is up to 15 years in prison.³

Involuntary Manslaughter

Involuntary manslaughter covers unintentional killings caused by criminal negligence or recklessness, or deaths that occur during the commission of a minor unlawful act. A doctor who prescribes a lethal combination of drugs without checking the patient’s records, or a driver who kills a pedestrian while running a red light, could face this charge. The defendant’s conduct was dangerous and careless, but it did not rise to the level of extreme indifference that depraved heart murder requires. The federal penalty is up to eight years.³

The gap between involuntary manslaughter and second-degree murder is where cases get genuinely hard to call. Both involve unintentional deaths caused by dangerous behavior. The difference comes down to degree. Was the defendant merely careless, or were they so reckless that any reasonable person would have recognized the near-certainty of someone dying? Juries often have to make that judgment call without clear guidance.

Defenses to a Murder Charge

Being charged with murder does not mean the prosecution’s version of events goes unchallenged. Several defenses can lead to acquittal or reduce the charge.

Self-Defense

A person who kills in genuine self-defense has not committed murder. The defendant typically must show they faced an imminent threat of death or serious bodily harm, responded with proportional force, and did not provoke the confrontation. At least 31 states have stand-your-ground laws that remove any obligation to retreat before using deadly force, while the remaining states impose some version of a duty to retreat if the defendant can safely do so.

When self-defense fails on technical grounds — say the defendant honestly but unreasonably believed they were in danger — some states allow what is called imperfect self-defense. It does not lead to acquittal, but it can reduce the charge from murder to voluntary manslaughter by negating malice.

Insanity

The insanity defense is raised far less often than popular culture suggests, and it succeeds even more rarely. The basic idea is that the defendant’s mental illness was so severe at the time of the killing that they should not be held criminally responsible. The most widely used standard comes from the M’Naghten rule: the defendant must show that because of a mental disease, they either did not understand what they were doing or could not distinguish right from wrong. Some jurisdictions use the Model Penal Code’s broader test, which also allows the defense when a defendant could not control their conduct due to mental illness. A successful insanity defense does not mean the defendant walks free. It almost always results in commitment to a psychiatric facility, often for longer than a prison sentence would have lasted.

Challenging Intent

Because different degrees of murder and manslaughter require different mental states, challenging the prosecution’s characterization of intent is one of the most common defense strategies. A defendant charged with first-degree murder may argue that the killing was impulsive rather than premeditated, aiming for a second-degree conviction. A defendant charged with second-degree murder may argue the death resulted from ordinary recklessness rather than extreme indifference, pushing toward manslaughter. These arguments do not deny that someone died. They contest why and how, which directly controls the severity of the punishment.

1
Office of the Law Revision Counsel. 18 USC 1111 – Murder
2
Library of Congress. Federal Homicide: From Murder to Manslaughter
3
Office of the Law Revision Counsel. 18 USC 1112 – Manslaughter