Is this some kind of joke? Slavery. Is. Evil.
There, I said what should have been obvious to anyone. Keeping another intelligent being on a leash, terrorizing it and calling it a "puppy" is a clear sign of an evil, and probably deranged, mind. Just because Elan was capable of using Thog's evil, bloodlust and lack of empathy to achieve a good end doesn't make Thog good anymore than Belkar's being used in the same way makes him good.