The Reign of Terror was a complicated sociopolitical occurrence that came about as a reaction to oppression but which came to adopt many of that same oppressive system's tactics, so its justification or lack thereof is largely a matter of opinion. It cannot be denied that the Reign of Terror was provoked, but moral positions differ on its rightness.
The excesses of the nobility in pre-Revolutionary France were a source of enormous frustration for both intellectuals and laborers. This boiling frustration led to the French Revolution and, in the chaos following that revolution, its leaders took extreme measures to enforce their ideology and pursue their personal vendettas. The violence that followed was extreme and by the Terror's end, over 40,000 people had been executed, many without trial.
The Terror did not pursue a coherent ideological line, nor did it allow for due process or even persecute a single group. Many of the people who lost their lives on the guillotine were not nobles or wealthy merchants but simply common citizens. This lends weight to arguments against the Terror's justification.
The Terror was a fundamentally reactionary regime. It pursued stability through violence and destabilized itself routinely with infighting among its leaders. In the end, it tore the country apart and paved the way for Napoleon's dictatorship.