I read this manga a while ago, a sage that followed a heroine (that he loved but never declared), make the demon king quest and when they return in triumph the king slander them with being corrupts and comdemn them to death. Despise the heroine be capable of resist she let herself be killed for the greatest good and the sage end dying with her, bot fall into a precipice where milions of ppl died before. The sage’s hate and the miasma of milions dead turn him into a lich, he make a comeback and destroy the kingdom and start to make war with humanity for the greater good, as only when pushed to extremes human beings are capable of show valor. I read the manga some time ago, but i don’t even know if there’s a novel of this. Anyone knowz?