Robespierre justified it because he said it enabled French citizens to remain true to the ideas of the revolution. Many radicals justified the use of terror by saying that is helped with control.
The answer is: “They sought to preserve the republic from its enemies”.
In order to preserve the French republic from the revolutionary tendencies of enemies, Robespierre and his clique justified the use of terror with his speech which was given in February, 1794:
“If the basis of popular government in peacetime is virtue, the basis of popular government during a revolution is both virtue and terror...”. So the aim of using terror was to protect the republic of France by terrorizing perceived enemies who planned to inspire a revolution and attack the rulers of the republic.