Just got this achievement without even trying to get it. :P
Actually I did:
- Looked around for every kind of secret in there (so probably I got all the flashbacks)
- Looted everything possible, even the bullets from the guitar player
- Didn't accept the bullets from Sasha's Mother
- Killed everyone in the "stealth" part
- Saved the Reds
Actually the Dark Ones in the end were saying that I must be stopped/killed etc, and I shot the last Dark One on top of the tower.
After that I just shot the tripod and got the good ending, reload, waited 20 seconds and got the bad one.