@criver We don't know the circumstances as to why the Hero killed the Demon Lord. Could be as simple as him being compelled to do it, or it could be that the Demon Lord was tyrannical towards humans. Perhaps the Hero wanted to save humans who were being invaded by the Demon Lord, but eventually the Hero came to care for demons. Could explain why he wasn't happy when he appeared in a flash back on page 4 since he condemned the entire demon race to ruin.