1. To commit suicide. Once John realized he had begun to have thoughts of killing himself, he decided to seek help.

2. To bring about one's death by performing a particular action. This phrase can be said when one's life is or could be in danger, but it is often used figuratively. I can hardly breathe—I'm going to kill myself if I go back on the treadmill right now! If they try driving home in this snow, they'll kill themselves—tell them to get a hotel room for the night.

3. To overexert oneself. Don't kill yourself trying to get the report done tonight—it's not due until next week anyway.