Sylvain Nsabimana was born on 29 July 1953 in the Mbazi commune of the Butare prefecture, Rwanda. On 19 April 1994, he was appointed préfet of Butare. He held this position until 17 June 1994, at which date he was replaced by Alphonse Ntezirayo. In this capacity, he was the representative of the executive power at prefectorial level. He held power over his immediate subordinates as well as the mayors and could officially call out the army and the national gendarmerie.
From end 1990 until July 1994, Nsabimana is said to have adhered to and participated in the detailed development of a plan aimed at exterminating the Tutsis.
Amongst other elements, this plan included recourse to hatred and ethnic violence, the training of and distribution of arms to militias as well as the drafting of lists of people to be eliminated. In the accomplishment of this plan, he is accused of having planned, ordered and participated in the massacres. From April to July 1994, Nsabimana is said to have publicly incited the population to exterminate the Tutsi population.
On 19 April 1994, the investiture ceremony for Nsabimana as the new préfet was the occasion of a large gathering of the interim government in Butare, during which the (interim) President Théodore Sindikubwabo gave a speech openly and explicitly calling on the population of Butare to participate in the massacre of the Tutsis. Nsabimana, reportedly, did not dissociate himself from this speech.
On 20 April 1994, Pauline Nyiramasuhuko is said to have requested military assistance to proceed with the massacres in the Ngoma commune. Nsabimana is said to have acceded to her request and to have given the order to the military authorities to provide her with the necessary reinforcements.
On the days following his investiture, Nsabimana is said to have called a meeting of all of the mayors of his prefecture. During this meeting he was reportedly apprised of the extent of the massacres against the Tutsis which had began in the communes of Butare. He took no measures nor made any decision to put an end to them. Furthermore, he is said to have dismissed those highly placed administrative personnel who had opposed the massacres.
Many Tutsis had sought refuge in the prefectorial offices in Butare. Beginning on 19 April 1994, the Interahamwe and the military, on several occasions, went there and assaulted the refugees. These attacks reportedly took place when Nsabimana was on the premises. Some of the refugees asked Nsabimana to afford them protection, but he is said to have stood by whilst the refugees were abducted, assaulted and killed on the spot.
Between mid-May and mid-June 1994, Nsabimana and Joseph Kanyabashi reportedly gave the order that the Tutsis who had sough refuge in the prefectorial offices be transferred to the Nyaruhengeri commune, in particular to Nyange. Following their arrival, they were attacked by armed men. Several of them were killed.
During May and June 1994, on orders from Nsabimana, refugees were said to have been transferred by the military, from the prefectorial office to the Rwandan Evangelical School (EER). They were subsequently taken to the forest close to the EER, where a large number of these refugees were beaten, then put to death by the military.
In the face of the advancing FPR (Front Patriotique Rwandais, an opposition group consisting mainly of Tutsi refugees and led by Paul Kagame), Nsabimana fled Rwanda. On 18 July 1997, he was arrested in Kenya and transferred to the International Criminal Tribunal for Rwanda (ICTR).