Anyone who is in denial about such a huge part of history does not deserve the rank of professor or the protection tenure provides. The genocide is well documented and generally accepted to have happened, and a worthwhile professor would be able to look at the evidence and come to the correct conclusion, instead of ignoring it all.
Even tenured professors are wrong. No one can deny cold, hard facts that Rwandan genocide happened between traditional African tribes that can't get along. Denial of Rwandan genocide isn't just distasteful, it's horrible because those deaths were completely preventable. If one of my professors denied something so heinous that was backed up by facts, I would drop out of school altogether if they didn't remove that person from a teaching position.
I believe if a professor knowingly teaches false facts in a class then they should most definitely be fired (and their tenure revoked) for doing so. This is one of the problems with tenure in academia because after it is acquired some professors stop producing good lectures and stop teaching as well as they had in the past.
No, the denial of a Rwandan genocide should not be grounds to revoke a professors tenure at a university. He has worked a long time to get that, and most likely is a very smart man, so his grounds for denying this are probably backed up by a good argument.
The professor's tenure shouldn't always be revoked. The denial of Rwandan genocide may be based on lack of evidence or opinion that doesn't come from a bad place. It depends on the professor's education and experience of the facts. The professor's abilities as a whole should be considered, as well as how this opinion would influence students.