I knew the author would mentioned why the ML became a cold-hearted bastard LOL. I mean who wouldn't, in their right mind, become a cold-hearted bastard to prevent insanity. He was abandoned by his parents understood that someone or a group killed them and was grieving. Then we see that most of his servants was probably lying and some taking advantage of his since he was a child (all cuz of his relatives), then people trying to kill him to take his inheritance from him since childhood until his becoming of age (he became an adult legally speaking).
Now, he is re-learning what it was supposed to be a normal human being so he likes to tease the MC cuz he feels that he can trust her but too much teasing duke!!! too much xD....although u_u can't blame you. when cute things look so cute when they are teased T_T you can't help it! you MUST continue the teasing U_U