As most of the pureblood wizarding race is gone, most of Slytherin house has to be of mixed blood. So what, pure by varying degrees? No, if it comes to a war between mixed blood adn pure blood, most Slytherins will side with the DA.
Side note - what if He-Who-Must-Not-Be-Named chose Harry because he was more afraid of Neville's pure blood and knew he might be harder to defeat. And in recuperation, is glad focus is shifted away from Neville. Maybe that's why he's allowed Harry to live time after time.