Murder the Clown is the stage name of Uncle Theo from the Goosebumps Most Wanted book, A Nightmare on Clown Street.
He tried to write a note to Ray but was caught by Mr Haha Face and sent to Clown Street, a prison for clowns that are abused by the true main villain of the book The Frightener. At the end of the book, Ray couldn't wash off his clown makeup on him. It is also assumed that Theo couldn't wash off his makeup as well and went insane as Murder the Clown.
The human version of Murder the Clown appears in the 2015 Goosebumps movie portrayed by Marshall Choka, but credited as "Monster #4." He is one of the villains and monster assembled by Slappy to wreak havoc in the town. Murder the Clown's appearance is much different from how he is pictured in the book. Murder doesn't speak in the movie, but his nose honked everytime he laughed. When the monsters are sucked into R.L. Stine's book, Murder the Clown briefly loses his wig before being sucked in.