Where's the line? What will it take to cross it?
I think the issue is that there was a line, and it got crossed. Once you cross it once, it becomes easier to cross, because hey it wasn't so bad last time.
Then, if you are put in relative isolation (enough for "group think" to take over) then it becomes easier still because you are validated for crossing it (dude we just saved lives by crossing the line...besides the "bad" guys are crossing it)
And this continues until you really can't even remember why you crossed it the first time, but there is so much danger out there you don't have time to really contemplate it, either. Until one day you realize that you are looking in the mirror each morning at someone who has become a stranger.
But by then it is too late...to challenge it now would precipitate an identity crises that isn't nearly as much fun as seeing yourself as the hero of the world.