It's the ultimate Harry Potter debate: Is Snape a friend to Voldemort or a friend to Dumbledore? Either way, Harry and his friends never trust the Potions professor who continually picks on them in class and is head of the rival Slytherin House. Plus, he's got a known passion for studying the Dark Arts.
Snape was a childhood friend to Harry's mom, but he never got over being humiliated by Harry's dad during their time as Hogwarts students. Though he now bears the Dark Mark that Voldemort's allies wear on their arms, Dumbledore seems to trust him.