Actually, they don't really seem to be any more evil than human beings are.
The dragon in Dragonshy wasn't evil, just... sleeping at the wrong place at the wrong time. He was actually a really sensitive soul.
The dragon in Owl's Well was pretty much just trying to protect his food source.
And Garble and his gang (Dragon Quest) were pretty much just bullies. And even then, Garble was a bit extreme compared to his posse considering their reactions when he actually tells Spike to smash the egg. So, even bullies have standards... it's just that peer pressure tells them to ignore those standards.