Spoilers for Act 2 and Act 3 follow.
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So in Act 2 I convinced Gareth to not kill his former Magister friend. Gareth decided to take him to a safe place. Not sure what happened to the friend, but I guess maybe he killed Gareth's parents?
I met Gareth at Paradise Downs, and I used my Barbarian tag to promise revenge. I went into the house and Gareth followed, but I talked him into doing what Lucian would want, so he decided to give up revenge, closing the quest.
At the start of Act 3,
Gareth is in better spirits, thanking me for setting him on the right path. He seems to be on the path that revenge sucks.
But fast forward a bit and I meet him confronting Alexander. Suddenly, he's back on the revenge kick, and I don't think my main, not a Persuasion-user, can talk him down.
Now okay, I'm certainly not the biggest fan of That Guy either, but it still seems a little odd to me to see Gareth completely reversing himself from literally the last time we saw him.