Criminal Law
The elements
A crime requires a voluntary act (or a legal duty to act), mens rea, concurrence, causation, and the prohibited result. The MPC mental states are purposely, knowingly, recklessly, and negligently; at common law, malice and specific intent do the work. Mistake of fact can negate mens rea; mistake of law generally does not.
Homicide
Murder is unlawful killing with malice aforethought - intent to kill, intent to inflict serious bodily harm, depraved heart, or felony murder. First-degree murder typically requires premeditation or an enumerated felony. Voluntary manslaughter requires adequate provocation; involuntary manslaughter is criminal negligence or misdemeanor manslaughter. Felony murder is limited by the enumerated-felony rule, the merger doctrine, and the res gestae requirement.
Inchoate and group liability
Attempt requires specific intent plus a substantial step. Conspiracy requires an agreement plus intent to achieve the objective; the overt act requirement and the bilateral-versus-unilateral split vary by jurisdiction. Accomplice liability extends to crimes the accomplice aids or encourages with the requisite intent; natural and probable consequences liability is jurisdictional.
Defenses
Self-defense requires reasonable belief in imminent unlawful force and proportionality. Insanity tests include M'Naghten, irresistible impulse, and the MPC volitional standard. Duress is generally no defense to homicide. Entrapment requires government inducement of an otherwise unwilling defendant.
Worked example
During an armed robbery, a clerk suffers a fatal heart attack. Felony murder applies if robbery is an enumerated felony and the death is a foreseeable result of the felony.
Rule of thumb: match mens rea to each element, not to the crime as a whole.