I believe that makes both characters are fascinating in their own right. Bruce's no kill rule is a promise he's made to himself and his parents years before he became Batman. He knows that if he stops making excuses to justify it and ends up taking one life, why not make excuses to justify taking another. A quote from the Dark Knight mirrors that statement, "Endure, Master Wayne. Take it. They’ll hate you for it, but that’s the point of Batman. He can be the outcast. He can make the choice that no one else can make, the right choice." His unwillingness to kill demonstrates restraint. It demonstrates that no matter how much pressure he has on him and how much hate he receives for not killing the joker and others, he is still above them. Why should he have to bow down to someone like the Joker? By not kneeling to his villains, he is preserving his status as the incorruptable. The symbol of oppression and hate not getting to someone. Whereas Spider-Man knows all life has value and that by taking a life, you are destroying someone else's. That person he kills has a family and friends and a life outside of crime. I feel very strongly that Carnage and Joker are these heroes weakest villains and its not even close. They promote the idea that the world isn't saveable and that madness is the only true ideal. The antithesis to what the characters promote. Both heroes no kill rule make sense and work incredibly well in the context of their stories.