After My Junior Brother Turned Black, I Went Crazy.

After my junior brother turned black, he turned out to be the demon lord who destroyed the world. He not only slaughtered my whole family, but also imprisoned me for a hundred years. After rebirth, I decided to strike first and kill this future demon lord before he awakens! But who can tell me why this junior brother was so pitiful before he became a demon? The eldest lady looks mean and arrogant, but is actually a kind-hearted little senior sister? In the early stage, she is a coward with low self-esteem, and in the later stage, she is a separation-anxious and wet man. Heroine: I will kill any villain or demon lord! Male protagonist: Senior sister doesn't like me, it must be my fault... Intention: This world is not a very good one, but being a part of the good will always make some things and people better.