As far as the elves are concerned, the humans ARE truly beings without worth or right to live, so by your own syllogism the slaughter is not evil.
You might have missed that I'm referring to an absolute morality. I'm not some silly relativist in real life nor in my fantasy games, for fantasy is traditionally a genre acknowledging the absolute existence of Truth, Good, and Evil, and I'll not betray that which is the very heritage and provence of fantasy as the Myth, Legend, and moral story of our day. Fantasy is so successful as a genre because it appeals to and does not shy away from our instinctive roots in and knowledge of Truth, Good, and Evil.


As for how extensive human hatred is in the canon, it seems pretty clear to me from the core sources that while there is a streak of Rhuobhe sympathizers in every elven culture, most elves have shunned him. Instead, Tuarhievel allows human guilders and makes friends with humans--not just the current Prince Fhileraene, but his father as well, and both reigns had more than a majority of support it would seem. Further, the Sielwode is not as anti-human as here suggested. Isaelie may have fits of rage, but it is said that she holds no ill will towards humans, and her lieutenant is a bit friendlier than that. Then we've got elves serving in Tuornen and living among humans in the Erebannien. No, I think elves are in the process of getting over humans, just as people and cultures who lose wars and loved ones gradually deal with their grief and anger and move on--or turn it to destruction (like the relatively fewer elves who join with Rhuobhe).