I don't get it. (spoilers!)

I don't get it. Starting with the lecture about the "YYY" chromosome. Can this actually happen?

Let's just say it can. Then why bother with the backstory (which took too long to get to) about the father murdering the mother?

If he kills people with his razor, why doesn't it cut his chest when he's being all erotic with it? (This really bothered me--perhaps he isn't really a killer?)

What happened with the girl on the path? Maybe nothing? It was kind of a coinsidence that it was the dude from the pet shop.

$30 for a hampter? This was awesome, only a lunatic would pay that for a pet that'll die in three years.

What happened after the hampster had babies? Was everything ok then?

If anyone can explain this movie to me, please do so. While I don't think it sucked, I think Netflix misreprested it....but I also wasn't sure what I was supposed to walk away with.