Mr. Kill-Kill, aka Mr. Murder, was first hinted in the end of episode 3 of The Little Buzzers, but was really starring for the first time in episode 4.
He appeared when the trio was beating to their own buzzing sounds and asked them to come with him. He explained that he wanted to make a record contract with them and be their manager. Sedrick was suspious of this who refered to himself as Mr. Kill-Kill, but the man insisted and showed them a contract signed "Yellow", "Purple" and "Green", claiming they had signed it already.
We later see them recording a song and later yet we se the trio confront Mr.Kill-Kill who is sitting in his office with piles of money, possibly from their music. We see him give them tickets to a concert with Lady Bass and the three seems contempt with that.
Mr. Murder or Mr. Kill-Kill is an intimidating figure who has an deep growling unsettling buzz, is considerably wealthy and makes deals with anybody who he can make a profit out off.