All I wonder now is what on earth happened to the man who could make such a speech, once he took power himself?

Mmm Robespiere wasn't quite what people usually think. Mostly war happened (his speeches against war are also very insightful, especially the part in which he foreshadowed the coming of the Tyrant aka Bonaparte). For instance few people know that the members of "Le Comité de Salut Public", including Robespierre himself, had to be re-elected by the Convention every week.

I think that something did happen to him at some point though, when he started losing the connection he had with the mass.