Face it, some Big Bads stay popular enough it might be a bad idea to kill them for real. So they may stick around or keep coming back, but too much of this can start to undermine a (super)hero's perceived effectiveness. While a work focusing on a single hero or group can Hand Wave this as perhaps what's necessary to end the villain for good being beyond their moral code, it can get more egregious if the fictional universe starts to get larger and more interconnected, and the villain keeps pissing off more and more people. This leads to the question of why some of those other folks with a lot fewer qualms against killing haven't put a bullet in them yet.