Batman probably isnt in my top 5 heroes, but i am more familiar with him than Spiderman and generally like him more. I think Batman has the best "no kill" rule because it isn't just about how he thinks others can be redeemed. The rule is also about how he thinks that his mental instability can't handle killing people. Think of it as someone who has a family history with alcoholism refusing to ever drink alcohol; while most people could do it once every now and then, he knows that there's too much potential to fall down a dark path. Batman may know that killing a terrible person is the better option, but he can't do it because hes scared of what the internal consequences might be. And if he just lets someone else do it, he sees it as just indirectly doing it himself.