You didn't knew, actually, who would? It just happened to be that you'd walked in on him, watching him suck on a poor innocent person's neck, drinking the blood out.
You froze in place and your eyes widen, he was a vampire all along? Your stomach twisted into a knot as you'd watched the disgusting scene. You'd held your stomach with one hand, your other free hand reaching to hold it over your mouth. You'd looked at the person he let drop, before back up at him. Wait, where was he? You hadn't time to reacted as you'd felt arms around you, pulling you closer until you were with your back against his chest. You'd looked up at him, with fear. But he'd reached a hand up, cupping your cheek, letting his thumb rub your cheek bone.
"Shh.. I won't hurt you, at least not too much."
he said, grinning widely as he bared his fangs, looking down at you before down on your neck, for him, it looked like a meal, something delicious to bite onto. He could guess your blood tasted sweeter than the blood from the person he sucked the life out.