@ Phelix
But people in real life did these things too. People enslaved and murdered countless people in Africa for profit, and literally killed anyone, including children, that they thought wouldn't turn a profit from selling. Britain got China addicted to opium and use that as a excuse to start a war. Yesterday I saw a account from a Russian soldier about east gemany, and he recollected that when they came upon a caravan of refugees the soldiers killed the men, raped the women and shot the crying children in front of the mothers, and all the soldiers, even the female telephonists, were laughing at this. Later, when they arrived at a village, the soldier saw two girls around 14-16, and told them to run away, but later saw that they were caught, after which dozens of soldiers took turns raping the two and then shot them in the mouth and tossed the corpses onto a pigsty. All this even when the 'other' is still human.
Hell, it's even going on today- the slave trade business is estimated to be the largest it's been in a while, and there are reports every few years about American soldiers being court-martialed for gunning down civilians for sport in the middle east (and imagine all the unknown cases of murder and rape from less open militaries like russia or china).
Besides, the elves' have only seen the bad part of humanity, of course their recollections will be bad. Since it seems that not every human is like that, only the soldiers/mercenaries(ie, the ones most likely to come into actual contact with the elves and beastmen) have shown to do all that stuff to the elves/beastmen.