Why did Robespierre think the Terror was necessary to achieve the the goals of the revolution?

Answer 1

He felt if he did this, it would advantage him politically.

Answer 2

Robespierre was a superb orator and opportunist who appears to have used the Nationalist sentiments of the time to further his own interests and career and to justify the murders of all who stood in his way. He was the type of man who could pay lip service to enlightenment principles while at the same time advocating the violation of one of its most fundamental goals: freedom of opinion. Such was his talent for manipulation of words. Though it was his murderous actions which eventually cost him his head in 1794, his words are what most betray his hypocrisy. In his speech "The Terror Justified", made before the French National Assembly in February of that same year, he explains that he felt he must "...lead the people by means of reason and the enemies of the people by terror..." -- Robespierre proclaims his dedication to "...virtue without which terror is murderous, terror without which virtue is powerless. Terror is nothing else than swift, severe, indomitable justice; it flows, then, from virtue." In spite of these strong words, the speech as a whole can be seen to have a touch of guilt in its tone; he was a man attempting to rationalize cold - blooded murder and he knew it. That he knew it is certain and demonstrable: he is condemned by words he himself spoke before The Terror first took post - revolutionary France in its grip.
It was used to protect and preserve the French Revolution.