Hi
I do not know if you can reverse hostility and be neutral again, but it may be the solution to repeat the last save when you got a mission "kill the traitor" and you kill him with a canister.
A canisters can sometimes be found in a few places in the game and you always have 1 or 2 with you, in this way, you can kill NPCs without becoming an enemy to others neutral or friendly NPC, just put a canister on the ground, through which the NPC you want to kill and shoot into the canister from a safe distance, when the NPC is next to it.