fromrequired, yes, initially Robespierre was for the rights of the poor and the ordinary people. And of course, he didn't turn into a dictator overnight.
But: Robespierre said, that Virtue couldn't exist without Terror - like, if you don't execute everybody who isn't virtuous (in a revolutionary meaning) enough, you can't have a virtuous republic, which they wanted to create. It was a very idealistic vision.
Also, Robespierre and his faction were forced to open the Terror by counter-revolutionaries and because of the pressure from countries (monarchies) in neighborhood.
Robespierre is considered being the most important person of the period of Terror (and they always go together, him and the Terror :( )

(Actually, your WWII studies could help ;) "later revolutions and dictatorships": their stories are, in some parts, somewhat similar)