The Belgian colonists imposed a social differentiation between Hutus and Tutsis that saw the former as mere labourers, and the latter as noble farmers. Subsequently predominantly Hutu governments expelled many Tutsis. Exiled in Uganda, these organised an army, the RPF under the tutelage of Yusevini and (after some infighting) the leadership of Paul Kagame, with the backing of the US and the UK. The RPF prepared for their invasion by assassinating Habyarimana, and sent Rwandan society into panic.
Already premissed upon racial exclusion, Rwanda's Hutu-dominated society attacked the Tutsi minority, on the grounds that they were loyal to the invading rebels. The slaughter was justified in racial terms.
The west did intervene. They systematically undermined the Habyarimana government, forcing them to take RPF representatives into government at the Arusha accord - concessions that did not satisfy Kagame.
Subsequently, the RPF in turn conducted pogroms against Hutu refugees from the RPF invasion, slaughtering thousands in the camps near Zaire. Kagame tested the patience of his Ugandan allies by his intervention in the (newly re-named) Congo.
In Rwanda, Kagame used the charge of genocide to systematically de-legitimate any Hutu representatives, with the consequence that the current government is overwhelmingly Tutsi, while former Hutu allies have all been jettisoned whenever they pressed their case. Kagame's 90+ per cent of hte vote in recent elections is an indication of the extent of the criminalisation of opposition in the country, less than it is of his own popularity.
Kagame's regime needs to keep public attention on the genocide to justify its minority rule.