No way. It is never said, anywhere in the books that they're all bad.

It's just that, obviously if your family are death eaters and such then of course you're going to be put into Slytherin so, and more than likely are going to become a DD aswell.

It has nothing to do with the fact that someone is a Slytherin, its who you are as a person. Other people who are put into Slytherin could just be really ambitious.

And what about Horace Slughorn, he was a Slytherin.