These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Acceptable Targets: The Nazi store owner, certainly. Probably a lot of other people that run afoul of Foster as well.
Definitely the violent gang members.
The first couple scenes treat the Korean clerk, the gangsters and the beggar as acceptable targets, then the scene with the Nazi deconstructs it as Foster's previous actions cause the Nazi to assume he's also a white supremacist.
Misaimed Fandom: There is a fanbase for this movie who think that Foster's actions were justified, including his abuse of his family, and believe that was the point of the film. Apparently they missed the revelation "I'm the bad guy?"
Moral Event Horizon: Many of his actions(fatally shooting the Neo-Nazi, who in his own words planned to send him to prison to be raped, going out of his way to non-fatally shoot someone who was trying to stop him) can't quite be called actions of an irredeemable villain. Smirking at a dying old man for hitting him with a golf ball was just plain sadistic and definitely showed he was starting to lose it.
One-Scene Wonder: The "Not Economically Viable!" Man. And the neo-Nazi freak is genuinely unsettling.
Strawman Has a Point: He's obviously extreme and very unstable but Bill's justification for his crimes (He devoted himself to a country that saw him as expendable and let itself rot with crime and social issues) is not hard to sympathize with.
Wish Fulfillment: It's very cathartic to watch this film after having a bad day, or year... You get to the end and think, "Well, I'm glad I didn't do that."
Iron Woobie: Sgt. Martin Prendergast, who is so constantly disrespected by his boss, co-workers and wife that he seems to forget all about it. But he's able to endure it without losing it and manages to find his self-confidence at the end.
Jerkass Woobie: Foster has an explosive temper and doesn't hesitate to use it. But he has been so beaten down by life and people around him that you're just amazed he doesn't go further.