My take on the essence of Slytherin is that they always put self-interest first--but what JKR refuses to acknowledge is that some few people can define self-interest as something larger than just one's self, imo. So yeah, staying on to fight against Voldemort and company is not really a good choice in terms of looking out for number one, but at least one person would have to think that, "gee, the world would be safer for me without the Death Eaters and Voldemort in it." There has to be a Slytherin Cedric Diggory somewhere in that house. It certainly wasn't Draco Malfoy, but there is someone.