I have Fane as my "main" character (not shape-shifted) and Gareth did not attack me through all of Act 1 and a few hours into Act 2. Do you think Fane has to be a companion for Gareth to attack him? Maybe there is something else causing him to be hostile (villain tag?).
Seeing as he screams something about the undead being on the lose, right before he attacks, it has to be Fane. Was the only undead party member.