That's because if the trauma of what they've gone through through the years of slavery from the Imperium, so of course they would hold animosity towards them because of it. And the fact if the natter is is that humans who are constantly killing, raping, and dehumanize elves that fuels animosity towards humans even more it would become pointless to coexist with them. And what gives the Chantry the right to interfere with the war between the elves and Orlais anyways? They should've stepped up to have peace talks and set up a treaty to benefit both sides but they didn't. They want the elves to fight so they could have an excuse to treat them less than human beings that proves otherwise.
Some dalish kill humans, which might not even know they're near a dalish camp, without second thought. It's not really much better.
The Chantry and Orlais have/had a extreme close relationship. Regardless, the Chantry isn't a foreign nation. It's an organization which has its center in Orlais, the country the elves were sacking.
Not that the Chantry didn't have their own interest in supporting Orlais over the Dales (other then the close relationship, their goal is to convert everyone), but I doubt I'd be that happy to try to make peace with someone who's invading me. Also, you're assuming the elves had any interest in peace, or would've listened to the Chantry as a peacemaker. Considering they were the first to actually invade the other, I doubt they have any intention to stop the war peacefully.
Besides, you're blaming the Chantry to supporting Orlais and not trying to stop the Orlesian-Dale war, and don't blame the elves to sit on their asses while the Blight ravaged other countries, a Blight which they should've known that it not stopped it would've cause harm for themselves as well?