He doesn't want to do this; he knows that both times he'd been unable to prevent himself from biting someone, it had hurt them. When he'd realized that Ned had the ability to heal himself, he hadn't felt quite so conflicted about his attack -- the conflicted feelings had come minutes later, when the inopportune kidnapping had occurred -- but when he'd attacked Charlie, he'd seen the resulting wound, seen the blood.

"I don't know if I can stop myself once I get started."

Because he's hungrier now, hungrier and angrier and those two feelings together make him think that preventing himself from biting down hard and drinking Charlie's blood to the point that it becomes life-threatening for Charlie won't be easy to resist. He shouldn't do this... but he can't tear his gaze away from Charlie's neck, either.