I love this song. I've always thought it was a pretty straightforward story - Guy drunk out on the town, runs into his ex, for whom he still has a lot of feelings, but also some resentment, hooks up with her, and decides either before or during the encounter that he's just doing it to get back at her, to break her heart. The individual lines are probably more open to interpretation and attachment of personal experience, i.e., "I assassin down the avenue" seems to me to be that feeling of "man, I'm pretty cool" that comes after a few drinks, with the use of assassin as a verb maybe showing t