**Spoilers**
Old Friends
Finish the game by sparing Letho.
In the Epilogue, you will walk through the city and kill a couple of guys before getting back to the front door. Save your game here. For this achievement, you must talk to Letho as much as you want to learn everything, then select "Let's Finish This" and choose to let him go. When he leaves, the achievement will unlock.
Reload your Save to unlock "Avenger - Finish the game by killing Letho."