You're the hero, and you've got a chance to take a shot at the villain. If you don't do it now, the chances are good the villain will get away, or the doomsday device will go off, or something equally nasty. One problem: your ally is in the blast zone. "Like You Would Really Do It", the audience (and maybe the villain) is thinking. An alternate way this trope can occur is when a character makes a Heroic Sacrifice by directing someone else to open fire on their own position in hopes of taking out the villain as the villain attacks them. Examples of Trial by Friendly Fire include: