How is Brutus portrayed as honorable?

In Julius Caesar, after Caesar is assassinated, Brutus addresses the crowd to try to reassure them that things are under control. He mentions that Caesar had honor but had to die because he was ambitious.
After some consultation with the co-assassins, he decides to let Marc Antony speak to the crowd as well. Antony, who was not part of the murder, had been close to Caesar.
Antony's speech is a masterpiece of combined praise and sarcasm. As he praises Caesar, he manages to undercut all of Brutus's criticisms of Caesar without ever overtly criticizing Brutus himself. Instead, Antony repeats a refrain about Brutus being honorable, piling them up until the audience can have no doubt that he means the opposite. For good measure, he includes the whole group of murderers.

Here, under leave of Brutus and the rest—
For Brutus is an honourable man;
So are they all, all honourable men—Come I to speak in Caesar's funeral.He was my friend, faithful and just to me:But Brutus says he was ambitious;And Brutus is an honourable man....
When that the poor have cried, Caesar hath wept:Ambition should be made of sterner stuff:Yet Brutus says he was ambitious;And Brutus is an honourable man.

Instead of contradicting the claim of ambition, he points out Caesar's sensitivity.
Finally, he reminds them that Caesar had the chance to be crowned king, and turned it down.

...he did thrice refuse: was this ambition?Yet Brutus says he was ambitious;And, sure, he is an honourable man.I speak not to disprove what Brutus spoke....

Again, he refuses to contradict or criticize Brutus, and keeps calling him honorable. Once he can tell the crowd is getting on his side, he insists he is not inciting them against the murderers. No, no, better to speak against Caesar and himself and the crowd. By excluding himself and them from the honorable men, he finally makes it crystal clear that he means just the opposite.

...if I were disposed to stir
Your hearts and minds to mutiny and rage,I should do Brutus wrong, and Cassius wrong,Who, you all know, are honourable men:I will not do them wrong; I rather choose
To wrong the dead, to wrong myself and you
Than I will wrong such honourable men.
http://shakespeare.mit.edu/julius_caesar/julius_caesar.3.2.html