Alucard doesn't like people all that much anymore...After he was betrayed he chose to stay isolated and away from the rest of the world...When you ran up to the doors of the castle screaming and begging for mercy he let you in...

It didn't take much for him to realize that you were being chased by villagers calling you a witch...He hid you away and treated your injuries, keeping you locked in the spare room so you don't have a chance to betray him...He's trying to keep you all to himself...