I agree with pickles2's interpretation of background based on the theme. That is, I don't think all those people were in his apartment.
From what I understand, it differs with the book and the movie but just talking about the musical, I think it was all a fantasy. The musical really pushes the redemption/sympathetic angle. Spoilers...
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With the song from his mother and stories about when he was young. And the way he rejects Jean to "save" her from himself. He's portrayed as someone who wants break free from his constraints and wants to effect change on these big issues (the rant he goes on at the birthday dinner) but ultimately, he just gives in and gets married to the woman he can't stand. Rejecting Jean (and not having actually committed any of the murders) redeems him but by not choosing her, he does not earn his full redemptive happy ending. It reminds me a bit of Peer Gynt, but that might just because I saw them so close together.