Homicide is a legal term for any killing of a human being by another human being. Homicide itself is not necessarily a crime—for instance, a justifiable killing of a suspect by the police or a killing in self-defense. Murder and manslaughter fall under the category of unlawful homicides.
Under the common law (law originating from custom and court decisions rather than statutes), murder was an intentional killing that was:
Malice aforethought doesn't mean that a killer has to have acted out of spite or hate. It exists if a defendant intends to kill someone without legal justification or excuse. In addition, in most states, malice aforethought isn't limited to intentional killings. It can also exist if the killer:
In today's society, murder is defined by statute rather than common law. Though today's statutes derive from common law, one has to look to these statutes for important distinctions—like the difference between first- and second-degree murder.
Even within the universe of those who kill with malice aforethought, the law regards some as more dangerous and morally blameworthy than others. First-degree murder applies to those defendants. Killings involving malice that don't amount to first-degree murder tend to constitute second-degree murder.
The rules vary somewhat from state to state as to what circumstances make an intentional killing first-degree murder, but the following circumstances commonly do so:
The killing is deliberate and premeditated. In other words, the killer has formed the intent to kill and has had time, however brief, to reflect on the matter. An obvious example of premeditation is a wife going to the store, buying a lethal dose of rat poison, and putting it in her husband's tea.
The killing occurs during the course of a dangerous felony. This crime is often known as "felony murder." Someone can be guilty of murder if a death occurs during the course of a dangerous felony, even if the person is not the killer. In most states, the death must be a foreseeable result of the initial felony. For example, assume that Aaron commits arson by setting a house on fire. Chris, a firefighter, dies while attempting to douse the burning building. Here, Aaron would be guilty of felony murder because a death occurred in the course of a dangerous felony, and the death was a foreseeable result of the defendant's actions. (Aaron might also be guilty of old-fashioned murder on the theory that he acted with malice aforethought.)
In most states, a defendant who didn't directly cause the death of an accomplice isn't automatically criminally liable for that death. Suppose, for example, that Bill and Janice burglarize a house. As they attempt to flee with the loot, a police officer shoots and kills Janice. Bill might not be guilty of first-degree murder, even though a death occurred in the course of a dangerous felony. (See Com. v. Redline, 391 Pa. 486 (1958).)
The killer uses an explosive device, such as a bomb. In California, for example, the statute defining first-degree murder specifies several ways in which the crime can occur. In part, it states, "All murder which is perpetrated by means of a destructive device or explosive … or by any other kind of willful, deliberate, and premeditated killing … is murder of the first degree." (Cal. Penal Code § 189 (2021).)
Many states have mandatory minimum sentences for murder. The mandatory minimum for first-degree murder is almost always higher than that for second-degree murder. Defendants convicted of first-degree murder can also be eligible for the death penalty. Many states and the federal government still have the death penalty. In those that do not, the maximum penalty is life in prison without the possibility of parole (LWOP). Defendants convicted of second-degree murder are often sentenced to a term of years rather than life in prison and are often eligible for parole.