The answer to this question can be quite extensive, for which I will suggest that you visit the Manitoba Historical Society page for further information regarding Louis Riel and his stay in the asylum of St. Jean-de-Dieu at Longue Pointe.
According to the records from the asylum dating back to 1876, Riel was admitted as a sufferer of what in modern psychology would be an episode of manic-depression. He was 31 at the time and with no previous history of episodes as intense as those that he suffered shortly before becoming institutionalized. By 1875, a year before going to the asylum, several things happened to him that rendered him in a very nervous state. First, he was banished from his own country for 5 years. Second, he had a bounty put upon his head for the many instances of revolt that he had led, and because the English-speaking Canadians had basically declared war against him.
By this time, his faculties began to fail him and he began to hallucinate and get ideas of grandiosity.
- diaries confirm that he saw himself as a world-class savior
- he truly believed to have been born with the Canadian mission mandated by something otherworldly
- he suffered bouts of sleeplessness and hysteria, crying and laughing alternatively
- he also suffered physical spasms and contortions, according to other journals, that clearly show that his condition was deteriorating.
The diary of Major Mallet details one incident where Riel is being transported and says
For the next six nights he did not sleep, cried, saying always that he was a prophet, and had a mission to fulfil. ... After four or five weeks he seemed to get better, still begging to go to church. I consented when he promised to act sensible. But he interrupted the Mass to contradict the priest. There was a commotion and I took Louis out. The priest called for silence saying that it was nothing, just a poor lunatic.
The episodes of manic depression alone do not necessarily contend that the man lost his mental faculties, but the megalomania, the ideas of grandiosity, his inappropriate behavior, and the fact that he could not physically control himself denote mental illness of a high degree.
Keep in mind that these episodes were culminating events, and not patterns in Riel's typical life. In fact, he must have been quite sane and courageous to lead rebellions, to found Manitoba, and to have inspired so many people to follow him. This means that, although Riel did suffer a major schizo episode, it was a long time coming due to the extreme situations that he had put himself into. Since there is no indication that Riel's parents suffered from mental illness, chances are that it could be a condition which underlies in Riel's biological make-up and was triggered by the combination of his own personality as it interacted with his chaotic environment.