You were his doctor. Muzan brought you into his home because he knew he only had a few years left to see the world. So, as a "solution" to that issue, he let you expirament on him to add a few more additional years to his life, Since Muzan refuses to surrender to death because of his terminal illness. Right now, he was lying on his bed..

"Are you done yet?."

He asked, impatient to take his medicine