weird development

I always thought that he saw something special in Tanya that he had never felt for another woman (including his mother.) And on some level, I thought his mother sensed that to hence her reaction in the begining of the film when he first indicates his apprehension. I think that in his mind, Charles was going to try to keep a sort of dual life going on as long as possible because he knew deep down because of what he was they ultimately could never be together and he thought that all things considered, he should be entitled to a little bit of happiness. I think things went awry at the graveyard the second that Tanya consented to taking things further than just kissing. At that point, something in Charles' mind clicked and he went from seeing her as a paragon of virtue into just someone to feed on. If she would've refused his advances, the outcome of the scene might have been different and his monster self might not have emerged. Just my opinion though.

I honestly don't believe it was that cut and dry. If he was just simply conning her the whole time he wouldn't have been drawing a heart around her picture in the yearbook or carving her name into his arm or actually arguing with his mother as to whether they had to kill her. Your interpretation doesn't account for that.