They’re [Slytherins] not all bad. I know I’ve said this before. I think I said it to Emerson - they are not all bad and— well, far from it, as we know, at the end— they may have a slightly more highly developed sense of preservation than other people, because— A part of the final battle that made me smile was Slughorn galloping back with Slytherins. But they’ve gone off to get reinforcements first, you know what I’m saying? So yes, they came back, they came back to fight. But I’m sure many people would say, well that’s common sense, isn’t it? Isn’t that smart, to get out, get more people and come back with them? It’s the old saying, there is no truth, there are only points of view.
J. K. Rowling on Slytherins.