First Degree Murder Defenses
Defenses to first degree murder charges fall into two major categories: claims that the defendant did not commit the killing in question, and admission that the defendant committed the killing, but did not commit first degree murder.
Defendants admitting to having killed the victim can assert defenses that they were justified in doing so (in self defense, for example), or that they were somehow incapacitated and thus not legally liable. These defenses require the defendant to put forth proof to support his or her defense.
First degree murder defendants also may simply argue that the prosecution has not proved all elements of a first degree murder charge typically that the defendant killed willfully, deliberately and with premeditation. Though the defendant may support such an argument with evidence, he or she is not required to do so, as proof of all elements of the crime falls on the shoulders of the prosecution.
As with statutes defining crimes, the defenses recognized for a specific crime can vary by state. Furthermore, which defenses a criminal defendant may have depends on the particular facts of the case in question. For guidance, defendants should consult an attorney well versed in his or her state's criminal laws.
In first degree murder cases, as well as other homicide crimes, defendants often argue mistaken identity i.e., that the prosecution has charged the wrong person with the killing. A defendant arguing mistaken identity often asserts an alibi if possible, which he or she tries to support with evidence of being somewhere else at the time of the killing. Other arguments in a mistaken identity defense include challenges to evidence placing the defendant at the scene of the crime. This can include challenges to witness identification as well as challenges to forensic evidence. A mistaken identity defense may also point to evidence implicating another possible suspect, but courts do not require defendants to do so.
Not all homicides are crimes, let alone first degree murders. The most common legal justification for a killing is self-defense or the defense of others.
To succeed, a defendant arguing self defense must show that the killing resulted from a reasonable use of force to resist a reasonable fear of death or bodily harm. The defendant cannot have instigated the threatening situation. The degree of force used in self-defense must be proportional to the threat perceived, and the threat perceived must be something that would place a reasonable person in fear of death or great bodily harm. Mere words or insults do not suffice.
The defendant's reaction to the threat cannot take place after the threat of death or bodily harm has passed. Many states require that the defendant attempt to retreat or avoid danger if possible before resorting to the use of deadly force.
For example, if someone incapacitates a mugger with pepper spray, he or she may need to attempt to flee to safety instead of taking out a pistol and shooting the mugger. States differ in the degree to which they require an attempt to retreat if the threat they face occurs in the defender's home.
Defense of Others
The reasonable and proportional defense of others also justifies some killings. The same requirements as self-defense typically apply: the use of force must be timely and proportional to the threat faced, and the perceived threat of death or bodily harm must be reasonable.
Exercise of Duty
Certain killings by law enforcement and other public officers qualify as justified homicides. If an officer kills someone in the exercise of duty and without an unlawful intent, recklessness or negligence, that killing generally does not constitute murder, let alone first degree murder.
Accident or Misfortune
Killings committed by accident in the course of lawful activities do not constitute murder. Some such killings may result in liability for manslaughter, but unless an accidental homicide takes place during the commission of a crime or as a result of other criminal intentions, they would not be covered by first degree or second degree murder statutes. In certain cases, such as parental discipline of children which results in even accidental death, the use of physical force beyond excepted norms can push the killing into murder and possibly, depending on state law, first degree murder.
Most states recognize an insanity defense to charges of first degree murder. Even states which allow the defense, however, treat it differently and often apply different tests. Most states define insanity, for purposes of determining criminal liability, as cognitively being unable to appreciate the quality of the act being committed, or unable to realize that the act is wrong. Some states also recognize a volitional aspect to "insanity" giving some defendants with disorders affecting impulse control access to the insanity defense.
For more complete information on how the insanity defense works and its different variants, see FindLaw's insanity defense resources.