How does cassius death contribute to the tragedy of julius caesar?

Cassius, the man who started the conspiracy, manipulated Brutus into believing that the Roman people wanted Caesar out of power. He did this by creating fake letters from "common citizens" and throwing them into Brutus's window to make him think a lot of people are against Caesar's rule. Because of this, Brutus decides to join the conspiracy, even though he doubts the act of killing Caesar throughout the entire play. In the end, Brutus deals the killing blow when stabbing Caesar, and tries to convince the Plebeians that this was done out of "love" for Rome, and not for power. Marc Antony, however, wins the crowd over and says what an disgrace it was for Brutus to kill Caesar because he was his closest friend and ally.