The Demonic Emperor
Full Title: Betrayed by the Hero Party, a Kind Person is forced to transform into a Demon, after which he schemes his Revenge and eventually becomes the successor to the Demonic Emperor.
Another stupid cliched revenge story. Warning: Gore (?), Mature and stuff...you know, because this is a revenge story, so expect high levels of violence.
Comrades with the Hero, Herman, and being proposed to by the Priestess Irene, who claims to love him...
Even though the members of the Hero party had their flaws and foibles, the ice mage, Klein Pearson, continued to fight alongside them against the legions of the Demonic Emperor, for the sake of humanity. For he is a "kind person" who wishes to protect the innnocent and minimize the casualties suffered by ordinary but stalwart human soldiers.
However, in order for the party to survive against a formidable foe, they sacrificed Klein and callously left him for dead, just so they could make their own escape.
With his legs chopped off and his route of escape sealed, Klein Pearson is left to face the wrath of the demons alone.
But...if he survives, he might just be able to become the next Demonic Emperor and command the scattered Legions against the "strongest Hero party."
Now then, let the revenge begin.