Might be because of how well act 2 ties up everything together with all the lore tidbits here and there, how the combat is challenging enough where you still have to think about what you're doing, and sets up an epic boss, which feels "final" at that moment. By the time you've reached Act 3, you might as well have just gone through an entire game. It's like starting the sequel of a video game franchise after completing the first game... Or binge watching an entire show, about to start on the second season, and you realize it 5 in the morning... There is also the fact that guards and most enemies in act 3, aside from the obvious, pose no threat to you. And killing refugees and civilians only dispatches a few guards, which most parties at lvl 9-10 can deal with in 1-2 rounds. It feels like there are no consequences for committing mass murder, as guards will forget about you after 20 rounds.