Génocidaires, from the French language for "those who commit genocide", refers to those guilty of the mass killings of the 1994 Rwandan Genocide, in which close to 1,000,000, primarily Tutsi Rwandans, were murdered by their Hutu neighbors. In the aftermath of the genocide, those guilty of organizing and leading the genocide (the Génocidaires) were put on trial at the International Criminal Tribunal for Rwanda. Those guilty of participating, profiting (e.g. through seizing Tutsi neighbors' property), etc. were put on trial in Gacaca courts.
It is also used more broadly to refer to any perpetrator of genocide.