revolution_fr | Question about Robespierre and The Terror

I don't have much knowledge about the French Revolution (as you can tell by looking at my userpic, I'm more of a WWII fangirl) but I'm greatly interested in it.

So in my AP Euro History class, we had to watch this documentary about the French Revolution. I'll post a part of it below:

I'm sort of lost because I thought Robespierre originally was for the rights of the poor and the ordinary people? It doesn't seem plausible to me that he can just turn into a sanguinary dictator overnight. Even in my textbook it says that Robespierre killed everyone whom he deemed unfit for his "Republic of Virtue," but history is never that simple. I know, I study WWII ;)

Anyways, can y'all people enlighten me about the cause of The Terror and Robespierre's role in it? Sorry if I'm asking too many questions.

EDIT: Here's the part that succeeds it. It basically describes the fall of Robespierre and says he inspired later dictatorships and revolutions.