The Reign of Terror was instituted during a time of crisis - France was at war, and it was going badly. French armies were being defeated by foreign, coalition armies - and inside the country itself, there was civil war in the Vendee with royalists/catholic counter-revolutionaries. In this atmosphere, the Republic looked like it was going to be crushed.

When the Terror was officially declared, in September 1793 - the Vendee was in revolt. The Federalist Revolt was in full swing (a reaction to the purge of the Girondins in the Convention), with the major cities of Bordeaux, Caen, Marseille, and Lyon in revolt. Armies from the rest of Europe threatened to invade France. The currency was in freefall.

Thus, the Terror was seen as the way to destroy the counter-revolution in France - the internal traitors, as well as to stiffen the armies of the Republic (using the Representatives on mission).

So the successful end to the Reign of Terror would have been a France victorious, internal enemies removed, and the threat of enemy invasion gone. It was an emergency policy, in a time of great need.

As for Robespierre's vision of government, I'm not qualified to answer (there's a biography of him that's on my reading list) - but the vision of him as the sole architect/driver of the Terror is not accurate. That's because it was highly convenient for the Thermidorans to lay all the blame on him, after the Terror was ended. Was he a hugely influential figure, and one of the poles of the Revolution, and support the Terror? Yes. But he was not the most extreme of those - and when you have characters like Fouche supporting it (who Robespierre reprimanded for being too zealous with his executions in Lyon...) - there's a disconnect. It's easier to ascribe all the blame on one person, especially after their death, than it is to think that the majority of a body as large as the Convention thought it necessary given the situation.

I'll quote an excerpt from David P. Jordan's The Revolutionary Career of Maximilien Robespierre (1985), p. 170:

To fight the war and simultaneously sustain the Revolution demanded experiment, improvisation, innovation. It seems unlikely that Robespierre was the sole architect of the so-called Revolutionary Government, but he was the man who most often came before the Convention and the nation to explain...His colleagues were forceful and intelligent and vigorous, and the majority of them were not acknowledged adherents of Robespierre's group nor even very fond of him. Still, he was the chief articulator of the theory of the new government because he expressed in his personal words their view of the Revolution. "The new government," Billaud-Varenne told the Convention on November 18 [1793], "will be terrible toward conspirators, coercive toward public servants, severe toward prevaricators, redoubtable to the wicked, the protector of the oppressed, inexorable toward oppressors, partial to patriots, tailored for the people..." Robespierre might have said the same thing: he had been saying it for years, which enhanced his prestige.

Robespierre was ahead of the curve in advocating the extreme measures that the Committee of Public Safety ended up implementing. Furthermore, his chief importance was as an orator and persuader in front of the National Convention; though he was personally disliked by even fellow radicals, his arguments and rhetoric were decisively persuasive for not only the Convention, but for the potentially incendiary Parisian populace.

Once he was called to power (or more precisely, power-sharing with the other Committee members), Robespierre's worldview and politics changed. Quoting again from Jordan, p. 171-2:

His earlier metaphors of self, his whole imagery, was gradually replaced by the vocabulary of war and armed struggle. His thought turned from defense to attack...His urge toward stark distinctions was fully satisfied: those who were not actively defending France were the enemy...Only in such moral absolutes was it possible to explain and apologize for the brutality of the fight...No attempt was made [by the disquisitions of Saint-Just & Robespierre of late 1793-early 1794] to disguise the Revolutionary Government as democratic. It was to be both autocratic and temporary. All authority continued to reside in the people, but during the emergency they gave up, so to speak, to their elected representatives all direct and immediate control over decisions. These representatives, in turn, made a similar sacrifice to the Committee of Public Safety. This "theory of revolutionary government," Robespierre argued, aware that he was breaking with his own democratic past and the democratic expectations of his followers, "is as new as the revolution that brought it into being." There were no precedents, either "in the treatises of political writers" or "in the laws of tyrants." New times demanded new responses.