I know there is a fine line it's really for the best the line remains for most heroes.
Kingdom Come, The Justice Lords from DCAU and The Authority (most of the time) are well written examples of what happens when you do things like that. Once the heroes start killing off villians then the game changes and they'll have to kill more and more often and it won't be long before the law stops tolerating them.
Further more heroes that kill are a concept that should only be allowed to be touched by very good writers. It is way to easy (as demonstrated by most of characters created and/or revamped in the 90s) to turn the heroes into remorseless psychopaths gunning down other remorseless psychopaths and that doesn't make for a good story.