He is a hero depending on the region of Canada and type of people you're referencing.

In Ontario, he is not a hero. In Manitoba and Saskatchewan, as he is portrayed as playing a pivotal role in the creation of those provinces and in that vein Metis rights, he is more often portrayed as a hero.

Even in these cases (with the exception of some very one-sided bad histories), Riel is regularly noted to have suffered from delusions of grandeur, likely mental illness, and to have basically degenerated by the end. Gabriel Dumont is much more typically treated as a hero since he was a far more practical (and effective) leader on the ground.

As for the general "how is he a hero?" well that's debatable obviously. But, take a look at a provincial map of Canada. Those who "like" Louis Riel will argue that Manitoba and Saskatchewan wouldn't be where they are (or at least would not have been where they were at the times) without Louis Riel.