The theme to Julius Caesar would have to be Evil action, though motivated by honesty, leads top disaster.
The conspirators where men who wanted the best for Rome. But they also wanted what was good for them. The plans and actions of the men where horrible ones. Their plans and their actions lead up to civil war and rioting. The opposite of what they had planned.
The evil action of the play is where the conspirators plot to kill Caesar. This is evil because they had no right to kill him, they just went on bad reactions to the people liking him so much. The plan to