One of the main ways that Shakespeare creates sympathy for Cassius is that Brutus always ignores his advice. This leads to their destruction. When Cassius dies, Shakespeare creates sympathy for him by saying it was his birthday.
Cassius is the one who brought Brutus into the conspiracy, but he pretty much immediately abdicated leadership to Brutus. The shame of it is that Brutus really did not know what he was doing. He was the nominal leader because his name brought dignity to the cause, but he didn’t have the experience or expertise. He led them into destruction from the beginning.
First there were the decisions about the assassination itself. Brutus contradicted Cassius even in small ways, such as on the issue of swearing an oath. He also had lofty ideals that did not translate into reality. This is why they did not kill anyone other than Caesar.
Our course will seem too bloody, Caius Cassius,
To cut the head off and then hack the limbs,
Like wrath in death and envy afterwards;
For Antony is but a limb of Caesar:
Let us be sacrificers, but not butchers, Caius. (Act 2, Scene 1)
Brutus’s biggest mistake was not listening to Cassius when it came to letting Mark Antony speak at Caesar’s funeral. This was a devastating blow to their cause. Cassius was afraid that Antony would sway the people. Brutus felt that his own speech would be convincing enough. Cassius was right, and Brutus was wrong. Brutus and Cassius had to flee Rome.
Cassius also did not want to meet Antony, Octavius, and Lepidus and their armies at Philippi. He felt that it was a strategic mistake. Brutus again overruled him, and this cost them their lives. Cassius committed suicide, on his birthday, when he thought that he saw his army being captured. It was actually a victory he saw. His side lost shortly thereafter, though.