You have every right to walk away. I wouldn't blame you in the least.
That being said, you may want to encourage GF to have her son evaluated by a adolescent psychiatrist that specializes in personality disorders. She needs to know what she is dealing with and how to deal with it. I say that with some experience.
A very good friend of mine has an adopted son that started showing some worrisome signs at a very young age. They tried many different approaches to correct his behavior, but nothing worked. I kept telling her she needed to have him professionally evaluated.
After the bad behavior continued for several years, my friend finally broke down and had her DS evaluated. He was a tween at the time. Some psychiatrists are reluctant to diagnose psychopathy (anti-social personality disorder) in adolescents and it often falls under oppositional/defiant disorder. This is seen as a potential precursor to full blown anti-social PD as an adult. There's a lot of debate about adolescent/adult psychopathy, and whether it can change over time (interesting article about this by Lynam, Charnigo, Moffitt, Raine, Loeber, and Stouthamer-Loeber, 2009, titled The Stability of Psychopathy Across Adolescence).
My friend's DS was formally diagnosed with oppositional/defiant disorder, and the psychiatrist that made the diagnosis told the parents that he expects to see it escalate over time and DS will likely be formally diagnosed with Anti-Social PD (Psychopathy) as an adult. He specifically told them their best hope was to just get him graduated from high school without getting arrested. He then gave them some guidance in dealing with him, at least to the extent possible. This child is a mess, and does some really scary stuff. BUT, at least they know what they are dealing with and can reach out to resources for help and support. At this point (he's 13 now), they are dealing with it a day/issue at a time (and they've got four other children as well).
That's why I say, before you walk away, since you have a 3 year history with her, talk to her to seek help with her son. She is going to need it whether you are in the picture or not.
I'm sorry you (and your GF) are having to deal with this. Watching my friend deal with this for the past ten years I've seen what I consider to be a brutal task in parenting for them.