I find your question highly interesting because for me, Act V is too late to look - you need to look back at the preceeding acts to consider the actions of Brutus, and most importantly, his mistakes, to trace his downfall.
However, if you are looking at Act V, it seems that his assessment of Octavius´ troops was rash and hasty, leading to a confusion which resulted in Cassius´ suicide because he thought all was lost.
If you want to go back and examine the issues that led up to this defeat, however, you will want to examine how Brutus systematically ignores or puts to one side the very shrewd advice he received from Cassius. Brutus refuses to recognise the importance of Marc Antony, telling the conspirators that he will not kill him along with Caesar, and underestimating his character:
for he is given
To sports, to wildness, and much company.
Then consider the permission that Brutus gives to Marc Antony to bear the body of Caesar and address the crowd. Cassius in vain tries to warn him of Marc Antony´s skills in rhetoric:
Know you how much the people may be moved
By that which he will utter?
This of course allows Marc Antony to sway the populace to his side.
Then consider Brutus´decision to march and meet the troops of Octavius and Marc Antony in Philippi in Act IV scene 3, again against the better judgement of Cassius. A tactical error which gave their enemies the advantage.
So, many mistakes, which makes us question Brutus and his character - why did he ignore Cassius so much? Was it his own sense of arrogance or did he genuinely feel that he new better?