Brutus is a tactical disaster. Crucially, he keeps getting major tactical decisions in the play wrong: and whether because of his idealism (that is, the belief that he is doing the right thing for Rome) or his self-regard (that is, the belief that he is doing the right thing for Rome) he doesn't seem to question himself at all.
The way to show that he commits these blunders for one reason or another is simply to justify in the text why you think he does it. I'd say he makes three major blunders in the play:
1) Over-ruling Cassius about killing Antony at the same time as Caesar.
2) Telling Portia about the conspiracy, leading to her suicide.
3) Over-ruling Cassius (again) in the tent-scene about the battle plan, which goes disastrously wrong and leads to both men's deaths.
I'll analyse the first-one to show you what I mean. Cassius makes a perfectly reasonable argument about murdering Antony:
We shall find of him
A shrewd contriver; and you knkow his means,
If he improve them, may well stretch so far
As to annoy us all...
No argument - Brutus just tells Cassius "no", because Antony is only a "limb of Caesar":
Let us be sacrificers, but not butchers, Caius.
We all stand up against the spirit of Caesar
And in the spirit of men there is no blood
Pure idealism. Brutus is proved wrong when the murder creates so much blood that he cannot ignore it, and so tries to make it an idealistic symbol - the hand-washing.
1.He over-rules Cassius as he tells Cassius that Antony is but a mere limb of Caesar and if one limb is gone it is like all the other are inactive.
2.When Cassius asks for Cicero to join in the conspiracy Brutus denies him of it.
3.The conspirators should have taken an oath but even that was denied by Brutus.
4.He should never have allowed Antony to speak at Caesar's funeral.
5.He should never have been so free with Antony.
6.He should have listened to Cassius when he was told to stay on the hill of Sardis,due to this both men loose their lives.
7.He should never have started the battle early because of this blunder poor Cassius was unable to cope with Antony and his forces.