At the end of Episode 3 you can make a choice,
Use Knife or take key and leave
This is for using the knife.
After doing this ending you can click continue and you'll be at the end again and you can easily pick the other ending for the achievement.
Some are having an issue where they can't continue after completing Episode 3. A good idea is to save before you make your choice like DeathIsland suggests in the comment below
I've made a 100% achievement walkthrough video for this game