The assasination of Caesar was justified, the thing that was not is the lacking of plan behind it. His Fellows were still alive and filled his Position, together with no restauration of the Republic to save it.

However if Rome should have stayed a Republic, it would have required a Reformation they have simply not done. They should have created small Parilaments for the Provinces rise Elections for Tribunes from those, aka introduce some Federalisation. The Fusion of Militery and political Power of the roman Provincegouvanors made them so dangerous and destructive for the Empire, like Caesar himself made abuse of his Power in Gaul.