This is a very complex achievement, and there doesn't seem to be a set of goals that you have to accomplish confirmed as of yet. However, one of the biggest things you have to do is make sure that you execute every semi-important character you can in the game. While it is unknown who needs to be killed for the achievement, here is a list of everyone who can be killed. Killing everyone is one way of guaranteeing this achievement.
Some people also say you can't meet up with Scarlet before the final mission if you want the achievement. To be safe, don't visit her.
Ali Shaheed: Can also be done right before the end of the game, assuming you don't kill him here.
Albatross/SIE: Depends on who you choose as your handler for the mission "Intercept Surkov at US Embassy. Must choose them over the data in the following mission, "Assault Brayko's Mansion". When you finally meet them, you can then kill him/her.
Championchik: Will encounter if you choose to kill Surkov in the mission "Prevent Surkov's Escape".
Surkov: See Price For Lying for more info.
Jibril Al-Bara: When given the choice between killing or spare him, use your sniper rifle to take him out.
Gelato Shopkeeper: As soon as you start talking, just execute right away.
Conrad Marburg: You must use Suave responses with him the whole way through Rome, see Youth Trumps Experience for more info.
Mina Tang: Make sure she does NOT like you. If she hates you, she will refuse your help at the end of the game. Stick with Professional responses, and when you're finally given the option, leave her behind.
Yancy Westridge: Encountered in every ending other than Crime Buster. After being fought, execute him.
Scarlet Lake: Make sure she does NOT like you. The more she hates you the better. Don't kill Leland off right away after defeating him or Westridge, and she will come by. Steven Heck MUST be your handler for this mission in order to kill her. When you are finally given the chance, give Heck the cue to kill her. If she really doesn't like you, Heck will kill her automatically.
Henry Leland: After Scarlet is killed, execute Leland.
Some users have posted their methods for getting this achievement, which can be found HERE, and HERE. Be warned, both links contain extensive spoilers from the game.