Why did the hutsi in rwanda hold a deep hatred for the tutsia?

During Belgium control of Rwanda during colonization the people of Rwanda were divided into two groups, lighter skinned Rwandans and darker skinned Rwandans. The lighter skinned Rwandans were called Tutsi, and the darker skinned Rwandans were called Hutu. As Tutsi were given preferential treatment over the Hutu by the Belgium, a resentment was formed. The manifested itself in the Rwandan genocide in which Hutu massacred Tutsi.