you think he would have gone straight if...

I'd argue that the film implies Max would have gone straight for a while, up until the point he was approached for a job by one of his old criminal buddies. We have two characters from the film who did exactly that until Max lured them back into committing crimes that ultimately killed them. We were introduced to the amateurish Willy, who had a family, imagined himself one day as a drummer, and was apparently being kept on the straight and arrow by his vigilant wife. We also saw the more criminally capable Jerry Schue, who appeared to have a fairly successful contracting business (painting, if I recall). He too had a wife that he claimed pulled him out of and kept him out of the crooked underworld. Just as the film Heat suggested, some guys feed off the buzz of criminal activity once they get started. In normal civilian life they're just one phone call away from getting the crime juices flowing again, much like drug addicts. This was especially true of Jerry Schue and probably Max Dembo as well. Max never had a chance thanks to his jackass parole officer.