We never really find out what happen to the girl right? Maybe John and his friend wanted to do the revolution peacefully, but then turned violent when the girl was kill by British during a peaceful protest. Maybe deep down inside John wanted her for him self and blames his friend for the girls death, and then Nolan betraying him was the last draw of their friendship.
shareThat could be, but then again maybe it is incidental whether the girl lived or died. I think Sean killed his friend simply because his friend was beaten into submission and betrayed not just "the cause", but him personally. I think it is fitting you never know David Warbeck's name (as this is a VERY cynical film), ie the idea that the ones who fight the revolution are always the ones who never get the glory the way the leaders do (do you know the name of the men who personally served with George Washington? Exactly).
At the same time, I almost felt as if it was (maybe) set up by Sean and his buddy, since his buddy not just nodded towards him (for the cops), but it also seemed as if he was giving Sean the sign to "go ahead and shoot". Also keep in mind the connection between Sean's friend being beaten into betraying the Irish revolutionaries and the doctor who was beaten into betraying Juan and their revolution.
Sean probably had to leave Ireland simply because he murdered 2 police men and would be charged with the death penalty.. so regardless whether his woman died, he would have to leave Ireland, assume a false identity (hence "John") and disappear.