To "spare someone's life" in Spanish (perdonarle la vida a alguien) is, idiomatically, to make a condescending gesture. I recently read that a Spanish critic "perdona la vida a Borges." Not a good thing. "Perdonavidas" means bully, someone who walks around with the attitude that if he isn't killing you he COULD do so if he wanted. What I like about the idiom is that it only has an ironical meaning. It makes no sense at all if not understood ironically.