It's been a while now, but. I think I was alluding to Booker's decision to betray the group; seemingly because he's fed up of being immortal and thinks Merrick might relieve him of this burden? That felt quite slight a reason to betray your friends and comrades of 100s of years.
[the whole "we have all this great tech but can't find Quynh; oh wait they show up at the end of the film" thing was a bit of a cheat too]