it seems unrealistic that all slytherins are bad, to me, too. I'm sure there's a few good ones in the house.
i think Draco is kind of an unrealistic villian, but i also think theres more to him than we see. I'm not a Draco lover who thinks he's good on the inside, but i do think that some of his nastiness is a cover up for something else. what, i don't know. insecurity perhaps.
I hope we'll get to meet some more slytherins in the upcoming books. there's bound to be a couple who aren't all bad, isn't there? Draco is definitely the leader of the nasty ones, and probably most everyone else sort of goes along with him.
Remember, slytherin himself was not always a bad man. he and gryffindor were at one time very good friends, but, slytherin did become corrupted. the qualities that slytherin had (ambition, resourcefulness) are good qualities, but are also easily corrupted buy lust for power, as we have seen in slytherin himslef, and LV.