Pacifism is one of Fallout GURPS disadvantages. See: Fallout GURPS.
You are opposed to violence. There are two different forms: Self-defense only: You will only fight to defend yourself or those in your care, using only as much force as may be necessary (no preemptive strikes allowed!). You must do your best to discourage others from starting fights. Cannot kill: You may fight freely, and even start a fight, but you may never do anything that seems likely to kill another. This includes abandoning a wounded foe to die. You must do your best to keep your companions from killing, too. If you kill someone (or feel responsible for a death), you immediately suffer a nervous breakdown. Roll 3 dice and be totally morose and useless (roleplay it!) for that many days. During this time, you must make a Will roll to offer any sort of violence toward anyone, for any reason”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Pacifism was removed from the game/recycled into Good Natured.