What decisions were made in Act V in Julius Caesar that lead to Brutus' death?

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?