I'm not sure. He doesn't like humans, but I don't think he's bad, just trying to protect his tree and the world's mana supply. He's angry at humans and half-elves for betraying him and causing the original World Tree to die. I don't know if I can really blame him for that. I mean, it did really mess things up for a long time.

[A very long time, and the world ended up being split in two (quite literally) because of it. It's pretty understandable why Ratatosk would be angry about that and why he would carry a grudge against humans even to this day.]