Why was the terror used during the French Revolution Justified?

Ok - I'll try to justify it.

The ruling elite, the Royal Family, the aristocracy and the land-owning nobility had been making themselves fabulously wealth at the expense of the hard working peasants. The Peasantry in France lived in utter squalor, as did the urban poor of the big cities; Paris, Marseilles and Lyon. The rich lived in palaces on great estates whilst the people were starving. The peasants toiled on land that their families had worked for generations could be thrown off the land at a moment's notice.

In the cities, the shop-owners, the lawyers, the doctors and other bourgeoisie felt that, as their endeavour was keeping the cities alive, and providing profit for the landlords, for the wealthy and the nobility, that they should have some political representation, and the monarchy should be limited in the way that it was in Britain.

That the King showed that it could not be trusted to allow the change to a constitutional monarchy he should be removed from the political scene. If he were exiled then he would be politically dangerous - he could raise an army and invade. If he were imprisoned, he could be used as an excuse for France's monarchist neighbours to invade, leaving death as the only option.

The Aristocracy similarly, could use their wealth and influence on foreign powers to invade - and they were against the revolution, so death for them as well.

In reality the Jacobins who instigated the Terror were political fanatics, bent on removing ALL vestiges of the Ancien Regime, from the head of State, to the church, to names of the months and days of the week - they began it because they thought the aristos represented everything that was wrong in the world - and so, they had to eliminate them.

As revolutions go, the French one in 1789 was among the worst. True, in the name of liberty, equality, and fraternity, it overthrew a corrupt regime. Yet what these fine ideals led to was, first, the Terror and mass murder in France, and then Napoleon and his wars, which took hundreds of thousands of lives in Europe and Russia. After this pointless slaughter came the restoration of the same corrupt regime that the Revolution overthrew. Aside from immense suffering, the upheaval achieved nothing.