Robespierre's actions are justified if the ends justify the means and that People's rights derive from the government.
Robespierre was obsessed with creating a society where everyone was equal and everyone had the same amount of wealth. To accomplish this goal he felt that any action or means was justified in achieving these goals.
People who had been associated with the previous government of royal privilege and wealth, were summarily executed. Their wealth was taken by the government to be redistributed. The middle class bourgeois who had achieve some degree of independent wealth who resisted the redistribution of their wealth were executed.
People who criticized the government of Robespierre were executed. Robespierre felt that the rights of the people were given by the government and could be taken away by the government.
Robespierre felt that these actions were justified by the noble goal of equality and fraternity in every manner.