Jordan -- regarding my statement about it being a morality tale, I think it is partly just a natural extension of the story being told as a first person narrative for a live, present audience that gives it that quality.
But to elaborate -- though its up for interpretation, I believe the murders are all complete fantasies in Patrick's head. They are his sole mental escape from the trap of the world he lives in. But in the end, when the fantasies are dismissed, Bateman is left with no choice but to acquiesce to the prison of that mundane world.
The lyrics to "This is Not an Exit" overtly state:
Even if this story
Is overwrought and gory
It’s not a fable, it’s not an allegory
No cautionary tale, no momento mori
But of course, like everything else Patrick says, this is a complete contradiction. The piece is an allegory, because, even though it is presented under the rouse of it being a satire of 1980s indulgence, the piece is actually still commenting on the pampered lifestyle of the average Broadway theatergoer / New Yorker, who has spent 145 dollars for their ticket and 50 bucks for overpriced drinks at intermission and needs to rush home quickly after the show to get up at 5AM to go work out with their personal trainer the next morning.
In other words, the musical brilliantly works as a morality tale because we believe we watch it as pure entertainment, not realizing that underneath its excessive symbolism, its actually (like the original production of CABARET) holding a mirror up, and commenting on all of us watching it.