"...therefore, the time light bodies take to fall, is the time falling bodies take to light."
--William Irwin Thompson, The Time Falling Bodies Take To Light.

When he quotes the church LAW, "till death do us part", why is he considered the crazy one?

Prendergast's wife comes across as even kookier than D-Fens.

he is a cop, allowed by the State to beat people and shoot people and rob people of freedom... while D-Fens is not supported by a ream of paperwork to acclaim his feral human instincts as worthwhile, therefore he is termed insane for enacting them.

non-military laity cannot even hit anyone who deserves our fist in their throat;

military - - presumed courage and bravado.

Prendergast has the audacity to assure D-Fens that he has 'a choice'. What choice?

And D-Fens took an option which most people would not even consider an option; they would call it 'a desperate last measure'. He got himself killed. This was one of the choices that the System (in the form of Prendergast) withheld from telling him about. As usual. But D-Fens was strong enough to see it for himself.

      He had to die, because he had removed himself so completely from the economic niche that he had been slotted into. And, we are told, having a job creates your self-esteem. No job, and you are NOT ECONOMICALLY VIABLE, therefore you have no niche in a society run on economics. But on this unusual afternoon D-Fens had explored the world as it should be, flying high, unshackled - a world which did not need money as a tool, but simply STRONG decisions - and found that when gravity inexorably forced him to re-enter the system, he would have to either end up a vegetable (for his new-found knowledge would far surpass any mundane job) or die as an exile. Falling down, there was no way he would remain just another cog.

      And just how bad were his deeds? First, he left his car in a traffic jam, because he and hundreds of others were not moving. Why stay with the car? If the council is going to okay construction roadblocks during peak hours, do they seriously expect people to wait for hours and risk breakdowns and injuries and waste money through repairs? So leave it. If everyone would co-operate (through realistic referendums and real civic planning) this construction crew would not be here at this time in the first place.
      Then D-Fens courteously asked for phone coins. But the store clerk was dishonest - lying that he had no change, when we know that every cash handler has change - and made the first move to attack him. So what would you do? You say you would not smash up the store, like D-Fens did, but is this because you feel the clerk acted correctly or because you are under threat of PENALTY if you do enact justice?
      Now here's the real crazy part: One would think that D-Fens would utilise his powerful position as antagonist - he is now the one with the baseball bat - and simply take whatever he wanted, but instead he pays for a soda after smashing up the store. In other words, he has displayed his 'craziness' by doing the right thing! Only if he did the wrong thing - steal - would you consider him NORMAL! Because that is what YOU would do! The majority.
     
      Next, he beats up two Latino gang hotheads. In his own words, he was willing to respect them and walk off what they considered their territory, but what set him off? Their greed. "Give us your fucken briefcase." He tried to impart reciprocal altruism, but the gangbangers couldn't comprehend this. He was the stronger, they the weaker. He triumphed. Who is really complaining? - except those particular bangers.
      And when they tried to gun him down, he survived and didn't even kill them in retaliation - again, because you would do this, you feel it is the 'correct' human reaction [who's the animal, you or D-Fens?] - simply shot one of them in the leg bemusedly. Survival of the fittest. Live or die.
      D-Fens offends a bum by giving him charity - but not in the way crippled people are offended when you ask to help them across the street - the bum was offended because D-Fens didn't give him enough to sate his GREED. This whining bum is representative of the attitudes of every 'have-not' who would ask unashamedly for support from the 'haves', thinking it is the DUTY of the 'haves' to dole out their earnings in charity  to the snivelling, lazy 'have-nots'.

The juvenility of the hamburger franchise - stopped serving breakfast, my arse! The lies of the pictured ads, the smallmindedness of the vacuous-smiley environment, the pitiful quality of the food; is it wrong for someone to actually come out and TELL THE TRUTH about these places? Who is complaining? Those whose lives felt threatened behind the counter, who would not have had to contend with D-Fens's anger if they were but educated with  real courtesy, not the brainless, perfunctory, prayer-like sales gimmicks that they are told 'courtesy' is.
      He shoots up a phone booth. Big deal. The council is insured against vandalism. Why did he perform what seems to be an irrational act? Some arsehole displayed discourtesy towards him, so D-Fens deprived him of the use of the phone. Don't you dare say this is extreme - when you have condoned your country's leader depriving millions of people of the right to FREEDOM because of perceived insults to the homeland.

D-Fens actually kills an Aryan Front Screwloose.

      He knocks out a ranting motorist. Who's complaining?
      When D-Fens talks to the bottom-runger construction guy about the condition of the street and government budgets, all he gets is an explosion of impertinent lip. (When the guy sees the GUN, suddenly he finds his respect.) Again, the council expects motorists to put up with this outrageous misuse of time, resources and taxpayers' money. A closed system, feeding on itself. They would not have to fix the street if they did not tear it up to make minor modifications in underground work that could have been done correctly in the first place. D-Fens's retaliation here is against the highest level bureaucrats who will never know the inconveniences their GREED creates.
      On the golf course, more discourtesy, with no concept of reciprocal altruism until the GUN is brought out. The golfer had no qualms in instigating violence, but as soon as he saw that the intruder was more powerful than he was, he backed down and showed respect. Natural selection acted out in miniature again. Best line in movie: "...and now you're gonna die wearing that stupid little hat..." 
      In no way did D-Fens terrorise the family barbecuers. They perceived him as a threat only because there were police after him - the majority creating the perception again.

taken for granted that he had some kind of mental problem, but everyone who interacted with him that day was even more twisted.