Complete Monster: The vile Neo-Nazi is perhaps the only truly villainous person in the film. This is a guy who brags about owning a used canister of Zyklon-B and threatens to rape Bill. He is the only person Bill intentionally kills and he really had it coming.
Designated Villain: All Foster wants is to get to his daughter's birthday but fate won't let him. And lets face it; pretty much everyone who pissed off Foster were selfish, greedy assholes and even more crazier nutcases. Foster attempted many times to reason with most of them until they went too far, forcing him to use violence to prove a point. He only actually kill one person and that person had it coming. Well, two if you count the old guy on the golf course (assuming he doesn't survive the heart attack).
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: If Foster doesn't crossed this already when he kills the nazi in the surplus store, or when he shoots a golf cart, causing one of the golfers to have a heart attack, then he might have when he shoots Sandra (non-fatally) on his way out to the pier and then appears as if he was about to kill his family as he tooks out his gun by scaring away the onlookers.
Nightmare Fuel: The story Foster tells to the family after he crashes their barbecue, that ends with he and his family "all sleeping together in the dark", which seems to imply that Foster intends to kill his wife, his daughter, and himself once he gets to them. The dreamy way he tells the story just makes it worse, as he genuinely seems to feel that is the right thing to do.
Foster's realization that he is in fact the bad guy.
I would have gotch'ya..."
Prendergast talking about his daughter and her death
The Woobie: Prendergast, who isn't treated well by his co-workers or his wife, and Foster. Even if he was unstable, you can't blame him and you just have to sympathize with him when you see how his country "thanked" him for his services.