All the characters on "Person of Interest" are fascinating in very different ways. Finch is the most mysterious character; he knows exactly what is right, according to him. Carter is the character who struggles with what she believes is right and Finch's version of what is right. Fusco straddles the line between what he knows to be right and wrong. And Reese blurs the line between right and wrong. Sometimes, he has no idea where the line exists. That is why he has Finch. But who made Reese that way?
Kara Stanton made Reese a killer
At the end of the episode "Prisoner's Dilemma," Kara has kidnapped Reese for nefarious purposes. The last time he had seen her before that moment, she had tried to kill him. Kara may have been doing the CIA's bidding when she killed people, but she clearly enjoyed her job. Her wide smile as she pumps people full of bullets in the "Prisoner's Dilemma" flashbacks showed that she loved her job to a disturbing degree. She made it her duty to turn Reese into the same kind of crazed killer she was.
Jessica Arndt taught Reese about love... and hate
Reese was in love with Jessica Arndt but chose a job over her, believing his job was important to the nation. Then Jessica married an abusive man who murdered her. Reese lost his love to evil. That alone could blur a person's sense of right and wrong. Reese's hatred for Jessica's murderer was enough to drive him to murder in the name of revenge and hatred. Was that the right thing to do? Reese didn't know anymore, falling into oblivion. His only hope was to be saved.
Finch saved Reese from evil
Reese was confused when Finch found him. His last life lessons involved being shot by Kara Stanton, having the CIA attempt to blow him to smithereens, and having the love of his life murdered by her own husband. His entire sense of trust, and of right and wrong, had been shattered. Finch's job was to show Reese that he could redeem himself. He could do the right thing while using his killing skills and paranoia as tools in the fight against evil, rather than succombing to it.
Carter is teaching Reese how to be human again
Carter is the most down to earth, normal person Reese has come across in a long time. Where Reese lives in an underworld of subterfuge and always-on surveillance, Carter lives in the real world. Sure, Carter is pulled into the Finch/Reese world as she works with the vigilante team, but it is not part of who she really is. At least not yet. She tends to pull Reese down to earth whenever they are together.