Ok so I have a theory: What if “But It’s Better If You Do”, “I Write Sins Not Tragedies”, “Miss Jackson”, and “This Is Gospel” are all one story?
Like Brendon has a boring life with his wife and he goes to illegal strip clubs where he falls in love with this one stripper who, as you know, ends up being his wife. They drive away in a car to the police office together and that’s “the end” BUT afterwards they separate because she cheats on him. Then, in I Write Sins Not Tragedies, she’s engaged to this other guy and Brendon goes to the wedding to warn him that “the groom’s bride is a whore”. The guy who’s getting married sees her cheating on him and becomes like Brendon, forever hating her. Then we go to Miss Jackson, where Brendon has tracked down where his former wife lives. She has company over, explaining her clothes, and as soon as they leave he kills her because of how angry he is. He then goes crazy because he can’t deal with the guilt of killing her and hallucinates, explaining the “zombies” and the other woman he beheads. We now go to This Is Gospel, where Brendon has been forced into a mental hospital. He fights against the nurses and doctors. Now this is where you have to use your imagination. He dies and ends up in a sort of limbo-ish type of place. This is where Hallelujah comes in. He wants to reach heaven but is weighed down by the guilt of murder. He eventually accepts it and hopes for forgiveness when he says “all you sinners, stand up, sing hallelujah”.