The Best Martial Arts Instructor In The World 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.nl fastest updating comic site. The Summary is

The Demonic Cult Leader, Seok Woncho, was a monster who even surpassed the Blood Cult Leader.Do you know who killed him?
“Ha Mu-baek.”
“But why are you bringing that up…?”
“Because he’s coming to our Mengryong Academy as an instructor.”
…Damn it. We’re screwed!
Ha Mu-baek, the greatest martial artist who crushed both the Demonic Cult and the Blood Cult,
is now assigned as an instructor at a martial arts training academy due to a political power struggle.
Determined to live quietly while keeping his identity hidden,
he soon finds himself solving the problems of cadets with heart-wrenching backstories,
disciplining the trashy human scum at the academy, and inexplicably becoming busier by the day…
Welcome! It’s your first time meeting a genius lunatic instructor, isn’t it?!