It is shown in the movie that Foster already had anger issues that slowly built up over time. But it is easy why they were building up:
In the film, it was shown he lost his job in the defense industry a month before. But it is possible that he was aware his career was on the line for a while, and the stress caused him to lash out at those around him, including his mom, who claimed he would give her a scaryDeath Glare at meals. The fact that he wasn't paying child support well before then showed he was trying to save as much as he could before he got fired.
Being forced from his home and living with his mother is dehumanizing for someone who is taught to be the breadwinner of his family.
Conversely, if his temper flared up, it could explain why he was downsized. Temper tantrums on the job aren't good for career advancement.
His Freak Out at the convenience store over small change seems pretty petty...but since he's been unemployed and unable to pay child support, it makes sense why he's obsessed over chump change. He would think every penny counts if he is flatly broke or at least too poor to support himself.
The fact that Foster's AC wasn't working in his car is also another subtle sign of his economic problems: considering how brutal Southern California's climate is, he probably would've paid to have it fixed by now.
The film seeks to portray Prendergast as similar to Foster, except that he learned to deal with his own aggravations maturely. However, Prendergast has something that Foster doesn't: better job security. Public service jobs, especially the police, are far more likely to guarantee steady employment, good benefits, and pensions. Also, police officers have the advantage of qualified immunity, which prevents them from being fired for any screw-ups on the job, which a private sector like Foster wouldn't be able to get. So yes, while Prendergast does have a right to call out Foster for his crimes, Prendergast can't understand Foster's economic problems since he has a steady income stream that at least prevents financial stress.
Foster's contempt for the homeless person is pretty understandable: Foster was, once upon a time, a hard-working guy who earned success and tried to support his family. He wouldn't have any sympathy for somebody bumming for change, especially someone who is spry enough to get some job and doesn't suffer the problems other homeless people might like mental health problems or drug addiction. It is also shown Foster is many things, but he wouldn't rob someone of their money.
Foster's sympathy for the "Not Economically Viable" person is pretty easy to understand: unlike the homeless man, the protestor wears a suit and tie and seemingly wants to better himself with a good loan. As a white-collar man forced into poverty, Foster can sympathize with a social aspirant being denied a path to success more than some bum who wants change.
When looking for boots, Bill is shown two pairs by Nick, one of which he recommends and one he doesn't. When Bill obtains his Ballistic Discount and changes outfit, we get a brief view of his new boots, which appear to be the expensive pair that Nick disliked. Bill probably took those out of spite, refusing to heed the advice of a Neo-Nazi.
When Foster goes to the fast food place, he argues and complains about how the food looks more appetizing in the ad than in real life. The reason for this is actually quite simple, the food in adverts is literally artificial because they are actually made of inedible substances, like plastic, so it looks more appetizing than it is. Intentional or not, the scene is a metaphor for how Foster would have just been angrier and more disappointed by the truth.
Considering how many guns those gangbangers had, and how they had no problem committing collateral damage while trying to kill Foster, one wonders what kind of crimes were they committing besides small-scale mugging? And what if they had weapons on them when they first confronted Foster? And if they're this murderous toward someone defending themselves, how many other people have they killed? Foster getting his hands on the weapons is (disturbingly) the better choice since while Foster is violent, Foster has something of a moral compass, and isn't looking to stick up a random person.
Considering how much of a hostile nutcase the Neo-Nazi was toward anyone who wasn't a straight white guy, not to mention willing to shelter (who he believes) to be a fellow traveler, what kind of things did he get up to in private?
Prendergast only figured out who Foster was because he happened to be driving in the same stretch of road Bill was and noticed his abandoned car. If he had taken a different route, he might not have been able to track down or catch Foster.
Was Foster actually planning a Murder-Suicide of his ex-wife and daughter? Considering at the beginning of the movie, he was willing to ditch his car and attack a cashier, he seems to be already behaving like a man who is about to die.
Foster's daughter is eventually going to learn that her temperamental father went on a rampage and may have been planning to kill her and her mother. On her birthday no less. Growing up with that knowledge won't be fun to say the least.
