this is a kid who apparently was a dab hand with dark magic when he entered the school, and wasn't afraid to use them.

But is that what we? JKR will reveal more, I think, but given that we see Snape being bullied by these people as children- a gang of four-sudeenly Sirius assertion of Snape being "Mr. Dark Arts" takes on the ring of Snape being a Star Trek Fanatic. In other words, a dork.