There is a line that any superhero must not cross if he is to remain a hero. This is the convention the major comic publishers have maintained since the 50s, and is now understood to the difference between a hero (no matter what other wrongs he commits) and a soon-to-be-villain-dark-hero.

But more to the point, this convention was established because no writer wants to definitively kill off a villian. If the hero kills a villian in plain sight, then you've got to have a damn good excuse to bring him back, and if you can't bring back villians, you're going to be hurting for ideas in a few months. Not to mention, any other author who wanted to use that villian is going to be hurting _you_...