This is one of the reasons why death threats online are so inadvisable. Not only is it rude and anti-social behavior, it also just doesn’t make any sense from a pragmatic point of view even if you were somehow threatening somebody that deserved it. You’re giving them a warning and flagging yourself to law enforcement, and opening yourself up to legal repercussions. It’s Lose-Lose and makes no sense to do unless you’re just a thoroughly unpleasant and anti-social individual with no intent to follow through, just wanting to scare people.