Cunning is often seen as a bad thing. There were many moments when we noticed Draco was always able to get what he wanted, even if this seemed unfair.
Some people would call that cheating, but that only shows you have a different perspective. Slytherins are known to bend the rules in their favor or even create new ones when necessary.
This isn't so different from their Gryffindor counterparts, except that their motives often seem more selfish. Regardless, knowing how to get what you want is a valuable skill and it's not something taught at any school. All the magic in the world can't give you a Slytherin's confidence.