It's a bit difficult to keep this spoiler free so I'll do the best I can, but also use spoiler tags...
This achievement is marked as story based but as I had to do two playthroughs, and was messaged by someone else who will now have to do a third, I feel like I ought to write a warning.
At the very end of the game, after picking up the last story item and going thru the dialogue with it, you will come to the last scene where you can only move your hands. You will have some dialogue and then have prompts to perform some actions. You will then have a choice.
Pick the "good" choice (I say "good" because this whole game has some questionable ideas about what's good and bad...). If you pick the "bad" choice then it's very probable that you won't get the achievement.
If that's not clear enough, here's the spoilerific version: