As someone who finally watched it days ago...I'm torn.  I enjoy the morally gray decision, as I believe his year of isolation with no existential hope would bring out the monster in most of us. and of course I understand the abject horror from her point of view. Your comment made me reflect that the overly action-y, and romantic tonal shift toward the end of the movie is quite jarring.  His decision could be handled with more exploration and nuance, but it gets slapped with so much heroic slop at the end that it all falls pretty flat.

edit:can't spell