I believe the trick with this sort of teacher will be capturing someone who is a equal-opportunity-hater, but also highly intriguing. By that, I mean salty old solders that hate recruits will still end up surrounded by them when he starts telling a war story. You also have to remember that your teacher will be dealing with royalty, so there's got to be more redeemable traits than not. Snape, in my book, had zero redeemable traits and was a lackluster character. This is just my opinion and the advice I would offer in the creating of such a character.