"Only our means differ" OP says, while ignoring that the empire's methods being genocide, brutal oppression, and slaughter of people who've already surrendered are kind of a big sticking point (and also ignoring that the empire doesn't "ally", it conquers). When most of a major power's leadership would apparently hear "war criminal" as a compliment, that means they're not ally material. Then there's the fact that ALL beastman tribes have good people in them (Brotherhood of Ash, Ehcatl, Idyllshire gobbies, and so on) and to endorse wiping out entire groups because of the perceived quality of their people is...at the very best, monstrous.