My personal thoughts on the standard "no kill" rule is that it doesn't quite work out but doing away with it completely quickly turns the characters into monsters.
I really think that superheroes should employ a manner of threat assessment to decide if they should kill the villain in question or not. In a similar manner to police procedure. I.E. if there are no possible non-lethal options for taking down the villain in a manner that reduces bystander and allied casualties to the absolute minimum then lethal force is acceptable. Otherwise the no kill rule stands.