Hahaha I love this song so much. Best song ever, although I think it is fairly weird. I mean, when I listened to it for the first time I was like 'omg, what the F. '. But it's so incredibly funny if you don't take it too serious lol.
But one more thing, someone wrote that guy in the songs kills the girl because he wants her in a sexual way, and doesn't want her as a person.
I think, however, that he wants to marry her because he loves her, but is afraid that she might not want him and then 'before the possibilities came true' he 'took all possibilities form you/her' by killing her, cause when she's dead she cannot reject him.
That's at least what I would think after watching the video, since he obviously proposes to her in that.
And the rest is pretty obvious, I think. If not - watching the video definitely helps.