I don't think he was a nutter before the start of the film. He was a family man, living the American dream, whose life had started falling apart. It shows what's inside all of us if we are pushed too far. He lost his job. He lost his family. He probably hated his job but it gave him a sense of purpose despite a horrendous commute every day. He felt lied to. Despite doing the "right thing" throughout his life he realised he was just there to be taken advantage of by everyone from his employer, his wife, thieves who felt they could just take what they wanted from him and even fast food shops which bait and switch great looking pictures of food for terrible quality food. It pushed him over the edge and once that veneer of civility had cracked, he allowed the crack to open wider and wider. That had started at the beginning of the film but he was a normal guy just a few months before that.
Robert Duvall could have gone the same way. But he found a career that did give him satisfaction and purpose and his marriage survived. That's the point of the film; to show that we could each crumble in the same way but some people are lucky to find satisfaction and support in their lives. Even the title of the film is about someone falling down, or tripping over. Before the trip they would have just been an average guy.
We've all had moments when we've almost lashed out at someone but managed to control ourselves. Now turn that frusration up a few notches and see if you can keep the same composure.
He felt lied to and had nothing more to lose.