Buttmonkey came from the same Military Primate Training Academy as Leon, but while Leon was excelling at his studies, Buttmonkey was mostly trying to bite other monkeys in the face. Buttmonkey was expelled after two semesters.
Outraged by his dismissal from the academy, Buttmonkey now has made it his mission in life to kill every monkey in the solar system. There are about 500 million monkeys in the solar system, so it’s a big task.
Buttmonkey eventually offers his help to Gibbs. Gibbs enjoys seeing people get bit in the face, so Buttmonkey amuses him greatly. Buttmonkey becomes Gibbs’ version of Leon; a tireless helper who does whatever lame tasks need doing. Buttmonkey knows it would please his new master greatly to kill Leon, and Leon is also now the second-most-famous monkey in the solar system, making him a prized target. (The most famous monkey in the solar system is Bib-Bib, star of a Mars reality show in which a monkey is made a Martian senator. Hilarity ensues.)