Deuteronomy 25:12 "Then you shall cut off her hand; do not show pity"

If we see a person who is determined to commit a murder, we must do all we can to prevent it.

If we cannot convince or stop him in any other way, we are permitted to apply force in our attempt to save a life (see Positive Mitzvah 247).

We may even cause him physical injury or kill him if that is the only way to stop him.

The Torah cautions us not to have pity on a person trying to take someone else's life.