I don't know that I go with that line of reasoning, since Elves are not pacifists and had killed before without becoming Dark Lords. No Elf could ever become like Sauron, because none of them have that kind of native power. He's a higher order of being than them, which is why his final destruction took so long and cost so much. Given that the Elves understand who and what he was, could one killing outweigh the eradication of such an enormous evil? Hasn't death been meted out in exchange for a desirable end before? I know that Elrond talked about corruption and such, but that was at the Council, three thousand years later. I always got the sense that it was a stance he arrived at in hindsight, after a couple of millenia of thinking about it. I don't know that it's something he thought or understood then, when it was happening.