I will be a badguy.

A good boy got killed. He reincarnated in a fantasy world as a demon lord descendant. He had the power of slowing time, cheat like defences,and knolage. He knew about Chemistry, Physics, Mathematics, Biology, History and Geography. He used his power, to became a lord of misery. Well for his enemies only. For his family and friends, he was demonlordly protective.
"You scared my cat?! Wanna die? haan?"