Victim of crime must have been alive, and also establish corpus delicti, or "body of the crime"... consisting of the fact that a human being is dead and that the death was caused by the criminal act or agency of another person, AND, with independent (direct or circumstantial) evidence beyond a defendant's confession (i.e., usually the body with statement of medical examiner to cause of death).
Defendants actions must be the cause of the victim's death... the defendant's act was the proximate cause of the victim's death... the natural and probable consequence of the defendant's unlawful conduct.
Death of the victim defined as "only" brain death and must occur within a stated period of time, though the one year and a day rule have been abolished by most states given today's medical advances. The reason for the rule was the inability of medical examiners to accurately determine the cause of death (as homicide) after long periods of time, which is not the case today.
Defenses may include heat of passion, reasonable care or accidental killing.