Basically, have him “resist the urge” every time you encounter him after he breaks his canisters, and when he finally fights against it, stay back. Visit him after this and he’ll join your party.
Doing this eventually unlocks the achievement that gives you a special tonic that uses your corruption rate to boost strength for five expeditions.
Of course, you could also keep tempting him to corrupt him fully. Once done, beat him five separate times to recruit a corrupted Torgar. Personally, I only bother to have him reject the corruption only once for the tonic, because an uncorrupted Torgar’s stats are ass. Corrupted Torgar isn’t much better, but at least he gives you a buff to your stats with his special trait, similar to Adel’s Timeless Bond. A maxed out Sub-Kanathar (the Imp) is honestly busted and can destroy almost every enemy in the game by himself, though I’ve yet to test on Nyx (the true final boss), because I can’t figure out how to get a non-corrupt eye to do the fight.