Morally speaking, we must realize that people choose their actions and therefore the people who actually engage in a set of actions are those who are most responsible for what they do. In the case of the Rwandan genocide, this means that the Hutu (and more precisely, those Hutu who planned and carried out the genocide) are the people who are most responsible for the genocide. It is possible to argue that other groups bear some responsibility, but ultimately, it was the Hutu participants themselves who chose to engage in genocide.
We can argue that the Tutsi bear some responsibility. For a long time, they ruled over the Hutu, stirring up resentment among the Hutu. In addition, it was specific Tutsi rebels who shot down the airplane of President Juvenal Habyarimana and who assassinated the Prime Minister. The Tutsi oppression of Hutus in the past, and their actions in 1994 certainly gave Hutus some reason for hating them.
We can argue that the Europeans and the world at large bear some responsibility. The Belgians ruled Rwanda for a time and only reinforced the divisions between Hutu and Tutsi rather than trying to reduce them. The UN and the international community as a whole did not do enough to stop the genocide as it was occurring.
All this being said, it is the Hutu participants in the genocide who are most responsible. Tutsi oppression in the past did not require them to commit genocide. Neither did Belgian colonial policy. It is also very difficult to argue that UN failure to stop the genocide excuses the Hutus who committed that genocide. Therefore, regardless of other factors, the main responsibility for the genocide rests on those who planned it and carried it out.