Can Muggleborn witches and wizards be evil? I had this argument with my friend, she said no, I said yes. I don't see why not, there are plenty of evil people in the world so I don't see why only Purebloods and Half-bloods would have an interest in the Dark Arts.
It's like saying Hufflepuffs can't be evil, they do have the least amount of Dark witches and wizards, but least doesn't mean none.