I've seen it 4 times. It seriously keeps getting better. Mind you I acknowledge many of the faults (story retread, Finn's programming mysteriously going awry, artoo mysteriously turning on, though apparently that was explained in the book, etc) but overall the movie was as good as i was hoping, and honestly better than it needed to be to get the Star Wars train wreck back on track. Loving the whole experience of this. Takes me back to seeing the OT in the theaters when they were released.
IIRC, even during training Finn had issues with Phasma with regards to his level of empathy. Since he'd never actually gone through a simulation involving mass murder (just battles), it's not unreasonable for him to start doubting.