Shakespeare doesn't give much credit to society as a whole when it comes to the ability to determine what is or is not ethical in Julius Caesar. The reactions of the citizens to the speeches of the conspirators in various parts of the play show that they are easily persuaded by words and do not take the time to think about what is or is not ethical. In Act III, scenes 2-3, Brutus and Antony each give their funeral speeches for Caesar. When Brutus gives his speech, the crowd is easily convinced that he did what was right and should be crowned the next ruler of Rome. However, when Antony takes the stage, he easily convinces them that Brutus and the conspirators were horrible men who should be punished for Caesar's death. The crowd does not take the time to decide what is ethical and what is not. Instead, they head toward the houses of the conspirators to burn them, killing an innocent poet on the way because his name is "Cinna."
If one wants to look at the individual's ability to determine what is or is not ethical, it is important to look at Brutus. From the beginning of the play, this character acts slowly and thoughtfully, and truly tries to do what he believes is ethical. When Cassius first approaches him in Act I, scene 2 about the idea of overthrowing Caesar, Brutus is not immediately convinced. The fact that he contemplates this instead of acting right away shows that, whether his ultimate decision is or is not ethical, he still has the ability to decide what is ethical based on the facts and what he thinks is the right thing to do.
In the end, Brutus decides to kill Caesar because he thinks it is the best for Rome. In his funeral speech to the crowd in Act II, scene 2, he says
If then that friend demand why Brutus rose against Caesar, this is my answer,--
Not that I loved Caesar less, but that I loved Rome more.
Had you rather Caesar were living, and die all slaves, than that Caesar were dead, to live all freemen?
This is a man who killed his best friend because he thought letting him live would lead to the enslavement of Rome. Brutus truly believes that what he did was in the best interest of Rome. Again, whether or not killing someone for the best of a society is ethical is debatable, but it still shows that he has the ability to decide and can put his own selfish interests aside to do something that he believes is in the interest of the greater good.