When she took a step backward, he knew -- somewhere deep down inside -- that he ought to stop. But then, "friend" or not, when she admitted to hugging Potter and said maybe he wasn't who she thought he was, Severus quite literally saw red. He wasn't the one hugging her enemies, after all.
"Maybe you're not who I thought you were, either!" he shot back. "Especially if you thought I wouldn't care if half the school knows my girlfriend is hugging other blokes!"