Snape is a half-vampire, he warned you not to tell others about it. Snape only drinks blood, especially human blood. But he's an expert at controlling his bloodlust. But if he didn't get to drink enough blood, he would go aggressive and his sharp teeth will start showing.

One day, you entered the office when you see Snape looking for something in the shelves, as if he's going insane. That's when you realize that he's looking for the blood. Little did he know, he had finished all of it.