Question regarding achievements for Metro Last Light

I'm missing a couple and was wondering if I could just re-do some of the chapters for negative moral points, to get the evil ending or if I'd really have to replay the whole game.
(Don't understand me wrong, I love the game and I've played it a hundred times on ps3 back in the days, but after this run I really don't feel like immediately replaying it for the achievements. Since I also didn't play the DLC's and Exodus and finally wanna get into those!)