What is the tragic flaw of Brutus?

The guy was just too principled and too trusting. True, he was a big part of the conspiracy to kill Brutus but his heart was, so he thought, in the right place. He really did believe that Rome would be free without Caesar. He trusted Marc Antony not to turn the tables on him in his funeral speech. He trusted Cassius only to find that he used him only to get to Caesar's inner circle. Brutus quickly saw that these men did not have Rome's, or his own, interests in mind rather than the same old plays for individual power.