How did the committee of public safety defend its brutal actions?

Answer 1

Always with the same cry: La Patrie en danger! France, at this point, was at war with the whole of Europe. In the national interest, nobody who might be a threat to national security could be allowed to live. As the whole of Europe was in arms against France to avenge the death of the King, it followed that anybody who might sympathise with the royal cause was a potential traitor. Similarly, as the Government (which meant, effectively, the Committee of Public safety) was prosecuting the war and defending the nation, any opposition to the Government, whether out of royalism or mere moderation, was potential treason.