Reading manga The Betrayed Hero Who Was Reincarnated As The Strongest Demon Lord at webtoon.
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 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
A Middle-Aged Hero, Betrayed by His Favourite Pupil, is Revived as History’s Strongest Demon Lord / The Betrayed Hero Who was Reincarnated as the Strongest Demon Lord / The Ex-Hero Who Resurrected as the Most Powerful Demon Lord / The Old Hero Who Died after Being Betrayed by His Student was Reborn as the Strongest Demon Lord in History / 愛弟子に裏切られて死んだおっさん勇者、史上最強の魔王として生き返る / 被爱徒背叛而丧命的勇者大叔，作为史上最强魔王复活