This achievement is the exact opposite of Judge, Jury, and Executioner. For this achievement, whenever you're given the chance to kill someone, you cannot. It's not a complex achievement to get, but you can screw yourself over very easily.
The characters in the game you have the option of killing, but must spare is as follows. Also, which responses work best with each person in order for them to like you for Thorton Inc..
Omar Nasiri: Extort works best. Unknown if Arrest will void achievement.
Favored Stance: None. Nothing will change his disposition of you much, just be sure keep him alive.
Ali Shaheed: Must be spared, never execute him when given the chance.
Favored Stance: Professional. You need to convince him that you are not in league with Halbech for his trust.
Grigori: Can't be killed. Try to get liked for Thorton Inc.
Favored Stance: Suave. Also, sending him the weapon shipments during "Investigate Weapon Shipments" will earn a like point with him.
Sis: Must be spared during the mission "Assault Lazo's Yacht and Retrieve Data".
Favored Stance: None. None of what you say really matters to her, as very rarely will she care. Should you get the mission "Contact Albatross", you will be able to use the "Locket" response. It isn't necessary in any way, but increases your rep with Sis, but lowers Albatross' rep by one.
Albatross: 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".
Favored Stance: Professional and Veteran, when available. Albatross doesn't like being threatened or having his time wasted, so stay Professional with him to get on his good side. Also, sparing Sis on the "Assault Lazo's Yacht and Retrieve Data" mission will earn a lot of like points with him.
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".
Favored Stance: Aggressive. See Savage Love for more info.
Konstantin Brayko: Must be spared.
Favored Stance: None. He doesn't care about Suave or Aggressive responses. Doesn't like several Professional responses.
Surkov: Must fulfill the requirements listed in Price For Lying. Then, you will have the chance to spare him by making a deal.
Jibril Al-Bara: When given the choice to kill or spare, spare him and abandon the mission. Unknown if killing him voids achievement.
Gelato Shopkeeper: Unknown if necessary for achievement, should spare to be safe. To be sure you don't kill him, follow the mission guide HERE.
Conrad Marburg: Stay Professional with him the whole way through. See Respected Enemies for more info. It is unknown if you need to turn him at the end of the game. To do so, you need around to get around 83% dossier info on him. When you meet him during the final mission, choose the "Dossier" response.
Hong Shi: Can't be killed. Try to get liked for Thorton Inc..
Favored Stance: Professional. If you collect all of the extra data during "Assault Triad Headquarters in Slums", do not blackmail him. Simply keep your end of the bargain.
Steven Heck: Can't be killed. Try to get liked for Thorton Inc..
Favored Stance: Suave. Going Suave, you make him like you very quickly. Also, deleting his data during the mission "Retrieve NSB Data from Grand Hotel" is a huge factor in gaining his trust.
Ronald Sung: Does not matter if you choose over Riot data or not.
Favored Stance: Professional.
Omen Deng: Must spare after boss fight.
Favored Stance: Professional
Alan Parker: Unknown if turning him is necessary for achievement. To do so, choose to talk to him during final mission. Once there, use Professional responses and when given the chance, use the "Analysis Flawed" response. Response only available if you've done at least one of the requirements(spare Shaheed, deal with Surkov, reveal Marburg's attempt on Madison, spare Omen Deng).
Favored Stance: Professional. Certain Aggressive answers in which you are blunt with him will also net approval.
Scarlet Lake: Possible to kill and still get achievement. To be safe, don't. See Exclusive Interview.
Favored Stance: Professional. Occasionally, Suave.
Mina Tang: Possible to kill and still get achievement. To be safe, don't. See Office Romance.
Favored Stance: Favored Stance: Professional to start, Suave after a while.
Yancy Westridge: Encountered in every ending other than Crime Buster. If fought, spare him afterwords.
Favored Stance: Professional, Aggressive.
Henry Leland: Encountered in all endings. Be sure to spare, no matter what.
Favored Stance: Professional, Suave(only when you can "Bluff" him, otherwise he'll dislike)
If you have spared everyone, the achievement will unlock upon finishing the game.