I disagree with that notion as well. There's some blanket statements being made about how comic books are for kids. But the reality is that while there are comics for kids, and while I fully admit that kids used to be the main audience for super hero comics, I don't believe for one second that that's still true. Honestly, I find most super hero comics these days seem to be written for adults like myself who grew up on them. It's still the characters we grew up with, but the stories and situations they're put in aren't as campy and silly as they used to be, and they can get pretty dark and mature at times.
But as far as Del's question of why someone like Batman would save the Joker if given the chance, I think I have to let my original argument stand. Choosing not to act if the villain does something to endanger their life, or is perhaps dying for some other reason, is as much choosing to kill them as if Batman tossed him of a building. Again, many of these heroes choose not to kill because they are after justice and they don't believe that they alone have any right to decide who lives and who dies. It's not a power they trust themselves to use objectively or responsibly, so they choose to leave that up to the system of justice their country/city/whatever has in place. Sadly, the problem is frequently that the justice system is inadequate to handle these super powered criminals and killers.
I almost have to wonder if it's an intentional statement on how ineffective the real world justice system can be as well. Probably not though.