This is a guide on how to get the alternate ending of Metro 2033, which will unlock the achievement. NOTE: This is how I got the ending, so in theory it should work for you too.

Actions to be performed:
1. Take the postcard from Hunter in Exhibition
2. Play guitar in your room at the exhibition
3. Give the father in the exhibition hall the balls he needs for his son.
4. Give a bullet to every beggar at Riga train station.
5. Give a scoop to the child who wants to lead you to bourbon.
6. Shoot the man next to the slaughterhouse in the market station.
7. Play the tape recording on the top floor of the building in Dead City 1.
8. Rescue the captured Reds in the Frontlines level, BUT ONLY KILL THE SURROUNDING FASCISTS.
9. Take the recorded tape from the leader of the Children of the Underground in Hole Station.
10. Refuse the payment the mother offers you for saving her son.
11. Send the tape to Polis via the transmitter on the roof of the fascist-occupied building outside Hole Station.
12. Listen to the first few men talking in the Black Station, BUT DO NOT KILL ANYONE ON THIS LEVEL.
13. In the vision, go directly after the Black Station to the Blue-Dark One.
14. Talk to Khan in the Journey to Sparta level.
15. In the vision, walk towards the Red-Dark One on the train to D6.
16. After your hallucination, shoot the laser target designator from the ledge.

Please take the opportunity to connect with your friends and family and share this video with them if you find it useful.