But we know without a doubt that tempering is a real thing. Brainwashing, as you described it. The Primals aren't just giving the beastmen more power, they're removing their individuality, taking away their freedom. This isn't a result of the corrupting influence of power, this is a supernatural removal of their free will.
But you're right, they do provide the beastmen with some power. However that's like making a deal with the devil. Sure, he might give you ungodly fiddling skills but he's going to take your soul too.
If what the Primals do was possible in the real world, I'm absolutely sure it would be considered the worst possible offense you could commit. It is essentially hollowing a person out and putting something else inside. If this isn't evil, what would be?