Europeans (Belgium and Germany) made an arbitrary map to split a region that put Rwanda on one side of a political line from its ancestral neighbors. This line separated people who had ancient ties to their neighbors and left them with less than they had enjoyed, with a different language, with different political views. When the Europeans could no longer plunder enough to sustain their occupation, they left with no regard to the turmoil they had caused.
Colonialism is directly to blame for the genocide in Rwanda. Not to mention the primary source of weapons used.