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.
Ah, the ol' Once You Pop You Can't Stop "Slippery Slope" argument.
What bullshit it is.
Sure we, the readers, can see that argument for what it really is (Joker Immunity masquerading as moral code). But from a reality standpoint, that "Thou Shalt Not Kill" argument simply doesn't hold up. As if killing off a complete monster villain somehow would transfer said complete monsterness to the hero. As if they will never EVER find themselves in a situation where it's the bad guy or them/innocents. As if non-super powered people (cops, soldiers, ordinary people defending themselves) don't deal with that every day without turning into The Punisher.
You can point at The Justice Lords and the Authority, I'll point at Vash The Stampede and Batman's Rogues Gallery as a rebuttal.