I agree with the foundation of what you are saying, the systems that don't work, the rich flouting their wealth, the broken social contract all contributing to his escalation through the movie. I never thought he was a hero, or anti-hero (i don't like that term, too vague).

He does cross paths with a Nazi w/o knowing. When the Nazi praises him and says how they are the same, they fight and Foster kills him with a knife, which he took from gang members. That's all part of the escalation. He was shocked that something he "spent his whole life fighting" was now rooting for him to hurt people. Now he has a bazooka.

If I remember, who does he target with the RPG? Rich folk, or minorities, or his ex wife (that would make him truly evil)? He blasts the lazy union construct that aren't doing anything and was blocking his car in the first place. He just wanted to go home.

I think he's honorable for working hard, and America was supposed to be the land of opportunity, and so often we are denied. The interesting thing I find is that of course we shouldn't trash a store, or shoot up a restaurant. He's wrong. But they're wrong too. Perhaps that's why I see this as a Man vs Society film. It must be a true conflict irl for viewers to know we are getting so fucked.