Partially quoted from enigk and the DVD commentary:
Five years prior to the time in movie, Bob's wife and child are killed. Bob moves into a smaller house, buys some fish as an attempt at relaxation and companionship, and begins going mad. (After the shootings, a news report mentions Coleman's wife and child dying 5 years ago. Also, when Bob tosses his first apple juice cap into the sink in his apartment, the window shows two hummingbirds -- symbolizing the dead wife and child).
Bob falls for Paula, but she treats him like crap and won't give him the time of day, so he creates wonderful Hula Girl Venessa and superimposes her over Paula's frame. Venessa was indeed Bob's virtualization of the Hula girl. She seems to appear whenever he comes contact with the Hula girl. His doll broke but her face remained undamaged, just like Venessa's body was paralyzed except for her face. (Also, later, when Bob takes Venessa to the office for a visit, she asks him where he was when the shooting happened and his reply was, "I was over by the Hula girl." )
After destroying his apartment, not maintaining the fish tank for a few months, and continuing his train ride to Crazy Town, Bob brings a gun into work.
As Real Bob is walking to the spot where he is going to open fire, he is imagining the outcome of his shooting 5 people and then himself (the beginning of his fantasy starts with the first appearance of Bob in the film, addressing the bullets). However, since he couldn't witness the events in a world without him in it, the shooter part of him gets personified as the Coleman guy (Coleman is an anagram of Maconel). This is also why Fantasy Bob drops the 6th bullet -- that was the one meant for him, hence, he lives in this version.
Bob shoots Coleman and becomes a hero, with nobody questioning why he had a loaded gun on him at work. (I wondered what the plane tire had to do with anything, but I agree with a previous poster who said that it conveniently allowed his alternate reality to lose the scrutiny of the media).
When Bob comes home for the second time and tosses the apple juice lid into the sink, there is one hummingbird in the window, because he thinks Venessa is dead. In Bob's house, at the wall we see a calendar which reads: "Today is 33". Since there is no month with 33 days, this is indicating that all that is being shown is fantasy/imagination.
The fish seems to represent Bob's anger/stress; he keeps feeding them.
Everyone who disliked or paid no attention to Bob all of a sudden loves him and invites him to play with them.
Bob gets a sweet (and completely ludicrous) promotion and a sweet car.
When Fantasy Bob is trying to decide if he will help Venessa off herself, he writes out: "Should I finish what Coleman started?" However, in bold type are the words "Should I finish Coleman?" Real Bob is beginning to question whether he is going to just shoot himself, as he is walking to the spot where he will open fire. (He's a very slow walker in my head, apparently)
Fantasy Bob gets the fantasy girl.
Fantasy Bob gets introduced to the grief counselor and instantly begins avoiding him. The grief counselor is the voice of logic, reason, and reality of Real Bob's mind. Fantasy Bob's enjoying all the imaginary attention, so he avoids the grief counselor like the plague. Grief counselor finally meets up with him and shows Bob the note, hoping he will accept that he is the problem.
The picture Bob painted on the head of a match was Icarus flying to the sun. It was Bob's real dream of flying away, yet, as anyone that knows mythology knows, Icarus's story does not not have a happy outcome. It was yet another clue as to where this movie was going: to represent a small and hidden aspect of himself -- his beauty & feelings were small and hidden.
At this point, Fantasy Bob sees a maintenance guy at the office sitting in his usual lunch spot with Bob's old briefcase and Hula doll. Bob follows the guy into the basement and finds a collection of "souvenirs." Symbolically, he maintenance guy is the sentimental side of Real Bob, metaphorically shut off into the basement by the death of his wife and child and his continual "crazification."
Bob reports the maintenance guy to the grief counselor as a person who may be dangerous, but the grief counselor traps him by telling him, "You're the problem, Bob" (going back to the reference "sacrificing himself to save the herd," Bob shooting himself -- the shooter -- would save the others).
Fantasy Bob's bubble bursts and reality seeps in. He gets in his dirty old car and metaphorically drives back to reality. (Fantasy Bob has lost the girl, the sweet car, the nice clothes, and the job -- boss's assistant doesn't recognize him.)
Fantasy Bob blinks out of existence and we cut back to Real Bob standing in his final spot. He looks up and realizes that there are no brightly colored snowflakes hanging from the ceiling as he pops back into reality. He sees Venessa flickering in and out, revealing Paula in her place. He lifts his foot to see the 6th bullet, his bullet, flickering in and out... It disappears from the floor, and is now in the gun, to be used. Bob realizes his voice of reason is right and that he is the problem. So -- kablammo -- down goes Bob. The imaginary Venessa, no longer needed, puffs out of existence into the form of a hummingbird.
Cops show up at ADD and take Bob's keys. Cops go to Bob's house and read his note, which was only neat and tidy in his fantasy. In reality it was wrecked.