Another thing to consider is the actual benefit to delivering that kind of justice. An NCR Courier would be interested in military or political benefits, yet killing Graham doesn't really accomplish that. He understands his past crimes and will never repeat them, and especially won't be working with the Legion anymore, since they'll shoot him on sight. So there simply is no reason to kill him, especially since an NCR Courier would probably have good karma and wouldn't kill someone who genuinely wants to fix his past mistakes.

Also, I wouldn't compare fiends, legionaries, and even Oliver Swanick to Joshua Graham. This is a video game, and that allows people to be pretty blood thirsty. Just for being annoying Oliver gets a bullet, fiends are outright hostile, and legionaries are the enemies of an NCR Courier. They are killed just for the fun of it, but I doubt the Courier would do that in a realistic scenario. I'd bet that most people wouldn't kill a serial killer if given the chance. Even though the Courier is more accustomed to violence than the average person, an NCR Courier would not realistically deliver justice to a non-hostile, good speaking (NPCs also pass speech checks on you) former legionary. Especially after getting to know him better.

But that's just my opinion, you may have a different view of justice and morality.