Remember that Grim Truth Sirius told when Dolohov first came the Hogwarts, about Rachel Brodie when she was just a little girl? He didn't kill her then. Maybe that was why.

It also might mean you'll have to REALLY watch out that you don't get caught snooping around his office. You're of age now, and that might mean that if he does catch you, he isn't gonna keep his wand holstered.