I don't think we have to overthink it. Maybe he was warned a bunch of times for inappropriate drug use. It's one thing to be intoxicated on your own time, but it's another when you are working. Maybe he was a huge jerk to a higher up during an audit or something and it was the last straw. Or, he has a sadistic streak now, and maybe he took some interrogations too far. He was becoming as much a liability as he was an asset. Corps had to check it's bottom line. I might rewrite that he was ex-government than Corps, making it easier for him to walk away and breaking law/over-interrogating could actually have consequences. Or maybe the Corps keeping him on for so long got to his head, so that he felt he was so important that he was valuable enough to go out on his own life mission so he burned his SIN and left. Playing a PC with these kind of issues is tough though, as you do have to work as a team. The character can be narcissistic and also know when to keep quiet or go along with a plan, for now at least...