There are two endings in the game. A morally good and morally bad ending. You will need to preform morally good actions to obtain the good ending.
This achievement is for the good ending. You will know if you have the good ending if at the end you regain control of your character and are able to shoot the laser targeting system. If you want this achievement then SHOOT
at it which will trigger a cutscene and unlock the achievement.
The obtain the good ending you must score a certain amount of moral points. However it is still unknown how this system works exactly so my advise is to follow the moral point list on the Metro Wiki which lists all the possible points and obtain as many as possible:https://metrovideogame.fandom.com/wiki/Moral_Points
As described earlier, you don't need them all but better be safe than sorry and try to get at least 80% of them.
There is also a different achievement for triggering the bad ending:
After you got the good ending and return to the main menu from the credits, press continue. Now play trough the game all the way to the final part. However this time DON'T SHOOT
the laser targeting system. Wait for the sequence to complete and this time you will receive the achievement for the bad ending.To obtain all achievements you have to play trough the game at least twice anyways so don't be annoyed if you get the bad ending on your first playtrough.