You could make a point of all them people that Hal (aka the good, oldschool Green Lantern) killed as Phalanx...but I guess he was a villain at that point...lol
I think Green Arrow doesn't have qualms with killing, but I'm not sure...
Azreal (aka the guy who took over for Batman during Nightfall)?
Marvel on the other hand, like mentioned by Master_Gio is chalk full of anti-heroes more than willing to spill blood...Punisher, Deadpool and Elektra SHOULD have been in the Vs. series! wtf was up with that!? Oh how I wish Capcom still had the Marvel license...Their Vs. series just hasn't been the same (though I still want to play the Namco and Tatsunoko ones)...*sighs and continues to dream of Marvel vs Capcom vs SNK vs Square-Enix*