Manadeshi ni Uragirarete Shinda Ossan Yuusha, Shijou Saikyou no Maou toshite Ikikaeru

The hero, Fried, confronts the demon lord with his student, Lyle. Fried killed the demon lord after a ferocious battle. "I'm going to put an end to this war!" Just as he was about to finish off the demon lord, he is betrayed by his student who attacked him from behind.

Fried, who was supposed to be dead, wakes up next to a demon girl who calls him the demon lord for some reason. It seems that Fried has risen as the new demon lord and will now have to protect the demonkind under his command while fighting his former comrades.

(Source: MU)