I got the impression when watching it that we were supposed to identify with his rants and this was what we were supposed to secretly be ranting inside anytime we encountered scenarios like that. It came off like it was supposed to be a black comedy that we were supposed to find funny because it let us vent safely the things we were supposed to identify with. Except I didn't. Many of his rants were stupid and unempathetic (to be fair I can only remember my impressions because it's been a long time since I saw the movie so I don't remember specific rants).
At the time, if you were a white person, you would identify with his rants, since most of the confrontations in the movie were riffs on things that older white people were always bitching about.
The difference is that the movie gets you to identify with the character, but it also sets up a tension within the viewer—who hasn't been exasperated by the stupid breakfast time thing (the movie amps up the absurdity by showing it's only 2 minutes past 10:30 and he can see breakfast sandwiches on the warmers, to really rally the audience), but then he pulls a sub machine gun in the hapless counter worker. But wait, that's something a crazy person does.
Or, the movie asks, who wouldn't want to finally show those cholos who's boss? Those people are really just thugs and cowards, all it takes is for someone to finally stand up to them and they'll just go away. But wait, if you think through this a bit, chasing people off with a baseball bat is kind of stupid and doesn't really accomplish anything, other than brief emotional satisfaction.
What's up with prices on shit going through the roof? Soda was like .50 a can a couple years ago and here's one of these fucking guys charging a $1.50? But wait, you don't go around smashing up the guy's store, that's kinda fucked up.
So as these scenarios play out, a tension should be present in the viewer who identifies with the common bitching points, but uncomfortable with seeing the fantasies play out and escalate in front of them. D-Fens is contrasted with the cop, who initially comes off as weak—he's quiet, he's too wishy-washy with his wife, etc. In any other context, we'd see him as a good-natured guy just doing his job, but being introduced after D-Fens—a man of purpose and action—he looks weak.
Eventually, the movie actually starts letting you off the hook by dropping hints during the cop's investigation that D-Fens has character defects (he's divorced, has a restraining order, doesn't pay child support). It allows you to start disassociating with him. You might share the same frustrations, but this guy is clearly has some issues, so we're not *that* alike, after all.
By the end of the movie, your initial views of the cop and D-Fens should've swapped. The cop is the calm, reasonable, mature guy who knows how to make compromises and get through life like an adult (WWII generation), and D-Fens is a caricature of the typical baby boomer who gets mad and throws a fit when the world isn't catering to them. The movie's basic judgement of D-Fens, and the people who sympathize with him, is: Yes, you're a bad guy, but you're also a baby who should've learned to suck it up and deal with it.