I have to combat the theories here that mercy and the need to censor death is for the sake of the children. It's true that comic books were more than likely created for kids, but I think that the need to be so good comes from something deeper that the writers sought for the sake of humanity instead and doing the best for your neighbor that you could, even if you're at odds with them. Also, they had to find a way to separate the heroes from the villians. What is something a villian would never do? Go out of his way to save a hero from dying.

Vivi summed up my thoughts pretty well.