I Am Really Not the 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.nl fastest updating comic site. The Summary is

Awakened from killing, to become the strongest demon lord! To survive, Fang Cheng is forced to sign a contract with the “Killing Devil”, and unlock skills by killing other devils, in this dark age under the invasion of devils, Fang Cheng swore to become the strongest, and kill his way towards being the strongest!