In most states, first-degree murder is defined as an unlawful killing that is both willful and premeditated, meaning that it was committed after planning or "lying in wait" for the victim.

For example, Dan comes home to find his wife in bed with Victor. Three days later, Dan waits behind a tree near Victor's front door. When Victor comes out of the house, Dan shoots and kills him.

Most states also adhere to a legal concept known as the "felony murder rule," under which a person commits first-degree murder if any death (even an accidental one) results from the commission of certain violent felonies -- usually arson, burglary, kidnapping, rape, and robbery.

For example, Dan and Connie rob Victor's liquor store, but as they are fleeing, Victor shoots and kills Dan. Under the felony murder rule, Connie can be charged with first-degree murder for Dan's death.