Ever since traveling with Lloyd, Zelos has tried extra hard to be more tolerant of other races. Half-elves, elves, dwarves- any other crazy race that's out there- they had a right to live how they want provided they weren't hurting anyone else doing so.

However accepting a people that eats them? Is possibly pushing Zelos to a level of tolerance that he (or any marginally-sane human, really) was not ready to jump.

"Are you kidding?! Of course it's still eating people! How many things do you eat while it's still alive- Wait no! I don't want to know! But I can tell you humans always eat their meat when it's dead, and trust me, it still counts as an animal!"

Okay, he was probably getting a little loud, but a line of frustration got crossed here.

"And don't talk to me about resurrection problems! That has nothing to do with not wanting someone to eat me!"