Was that the game where the Avatar is acting out of character and destroys a culture or was that the next game?
That was the one after Serpent Isle, Pagan. Though, I
think the jury's still out on whether or not he actually destroyed Pagan or just kinda...wrecked it, since soon after freeing the titans, I think the Avatar basically ate them. Two were outright malevolent (Hydros and Pyros) and likely would've done some damage in the meantime between being freed and being eaten. Though, the whole idea behind Pagan was a question of whether or not its good to countenance a lesser evil in service to a greater good. The Guardian had long since taken over Pagan, any resistance to his ideologies were wiped out hundreds, possibly thousands, of years before the Avatar even arrived there. Its magic was largely (minus Thaumaturgy, which existed outside the realm of normal Pagan magic) channeled thru the titans and they were considered the source of said power. Pagan was ruled by either secretly malevolent (in the case of Stratos) or outright evil (in the case of, again, Pyros and Hydros) cults whose interests aligned with the titans they served. The world was basically fucked long before you got there...if anything, you could see the Avatar's actions as hastening the inevitable. It also served as an object lesson in just what the Guardian had planned for Britannia and all worlds after that, which not only made the Avatar want to leave that horrible place just for the sake of leaving a horrible place, but also to stop the Guardian from doing this to any other worlds. Freeing and eating the titans gave him the power he needed to confront the Red Bastard on his home turf which, incidentally, was the original plan for Ultima IX: for the Avatar to go to the Guardian's home world and kick his enormous, crimson ass.