You ask more than one question in your question. You should break these up and ask one question per question and you'd be more likely to get immediate responses.
I'll deal specifically with why Brutus is considered the tragic figure.
Brutus makes poor decisions. He is nearly an ideal Roman, and he thinks others are, too. He is naive, when it comes to judging other people. His poor decisions lead to the civil war, the failure of the conspiracy, and his own downfall.
First, in Act 2.1 he chooses to join the conspiracy, believing that the other conspirators have the same honorable motives for wanting Caesar dead that he does. They don't. They are motivated mostly out of envy and self-interest.
Second, also in Act 2.1, as the most respected member of the conspiracy, the decision is his and he decides not to kill Antony along with Caesar. This is a mistake, since Antony will deliver the speech that will snatch control of Rome from the conspirators and ignite the civil war.
Finally, concerning Antony's speech, Brutus makes the decision to allow Antony to speak at the funeral (Act 3.1). Cassius warns him not to let Antony speak, just has he warned him to kill Antony at the same time Caesar is killed. But Brutus decides against Cassius both times.
These decisions lead to the downfall of Brutus and the other conspirators. Their fate is in Brutus' hands. He is the tragic figure.