'Traitor' is an interesting word to use. Brutus was clearly NOT that, but was, in fact, a patriot trying to prevent the usurpation of power by Caesar, and the destruction of the Republic. Of course, it was futile - Octavian ppbecame Emperor, not just King.
No, he (along with the other conspirators) were afraid that they would lose power if Caesar was allowed to continue consolidating it in himself. They were attempting to keep their own stranglehold on power. It just so happens Brutus was a little more idealistic than the rest.
Anyway, if Brutus doesn't join the conspiracy, it almost surely can't be successful. If he doesn't join, I presume his older brother Decimus isn't joining either? Anyway, part of me doubts the conspiracy would actually amount to anything if they don't join, but if it does, they'd likely warn Caesar ahead of time, or at the very least not stop Antony from informing Caesar like they did on the Ides.
I believe Marcus Brutus (or Decimus, again I forget which, Caesar had an affection for both of them) was also almost an equal heir to Octavius in Caesar's will. Or at the very least, was supposed to be the main beneficiary in his will if Octavius had died before Caesar (which he very well could, given his tendency to get deathly sick).
So if Brutus doesn't join the conspiracy and stands by Caesar, Caesar lives and goes on his campaign as scheduled.