The Betrayed Hero Who Was Reincarnated as the Strongest Demon Lord is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

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