Myst, Might, Mayhem is a Manga/Manhwa/Manhua in (English/Raw) language, Action series, english chapters have been translated and you can read them here. You are reading chapters on-site. The Summary is

Jeong, the Slaughtering Scythe Demon, feels joy in killing others. He happens to look just like Mok Gyeongwoon, the third young master of the Mok Sword Manor… “If you pretend to be me and live my life. I will let you out of this prison.” “Will you really give me that chance?” Slash Kill the third young master and live his life pretending to be him. Will the fake Mok Gyeongwoon survive this life? The serial killer begins his new life as the third young master of the great Mok Sword Manor.