This is a generic reincarnation story.

After dying at the hands of his loved one, a lost soul caught the attention of The Manager. Taking pity on the soul, The Manager allowed it to have another chance at life in a different world.Bearing a near-insane hatred towards his fellow men, the reincarnated soul decided to go on a quest for vengeance against the nations of men, bringing with him armies of monsters and demons to destroy all civilization. However, before he could finally obtain his revenge, he was defeated by a hero.After dying a strangely peaceful death, The Manager once again makes him an offer. Since he entertained The Manager enough, he is once again granted another life, however, this time it had a twist: he is now a girl.