In September 1992 he became a [[pentito]] and started to collaborate with the authorities. He claimed he was disillusioned by Riina's tactics of murder and treachery, complaining he had been used then discarded by Riina. Marchese admitted to have participated in more than 20 murders, including the one on [[Stefano Bontade]] and [[Salvatore Inzerillo]]. He was the first mafioso from the winning faction of the [[Corleonesi]] in the [[Second Mafia War]] to become state witness.<ref name=narco/>
Giuseppe Marchese was ana brother in-law of [[Leoluca Bagarella]] from [[Corleone]], who married his sister Vincenzina in 1991. According to [[pentito]] Toni Calvaruso, Vincenzina committed suicide on 12 May 1995, due to her depressive state after a series of miscarriages, her brother becoming a [[pentito]] and her husband's involvement with the death of [[Giuseppe Di Matteo]].<ref>{{cite web|url=https://ricerca.repubblica.it/repubblica/archivio/repubblica/1996/01/17/la-moglie-di-bagarella-si-impiccata.html|title=LA MOGLIE DI BAGARELLA SI E' IMPICCATA|publisher=repubblica.it|date=17 January 1996|language=Italian}}</ref><ref name=longrigg122>Longrigg, ''Mafia Women'', p. 122</ref><ref>{{in lang|it}} [http://www.psychomedia.it/pm/lifecycle/gender/flocca-giunta.htm Il femminile in Cosa Nostra], Francesco Flocca & Serena Giunta, Psychomedia, September 5, 2003</ref>