CON:

• In retrospect, of course, the joining of Brutus in the act of conspiracy is unwise. In the final act, at the battle of Phillippi, Brutus has his forces attack those of Octavius and they are defeated. Of course, after his death, his successors, the triuimvirate of Octavius, Antony, and Lepidus become much more tyrannical than Caesar has been.  For, the only evidence of corruption on the part of Caesar is his having Flavius and Marullus is their "pulling scarves off Caesar's images."  Other than this incident, there is no evidence of Caeasr's tyranny.
• Brutus allows himself to be deceived by Cassius. For, Cassius's motives for enlisting Brutus are purely selfish:  (a)  He forges letters from the Romans that "testify" to Caesar's acts of tyranny, and he leaves them where Brutus will find them. (b) He flatters Brutus,

...you have no such mirrors as will turn/Your hidden worthiness into your eye,/That you misght see your shadow...I, your glass/Will modestly discover to yourself/That of yourself which you yet know not of....honor is the subject of my story (I,ii,56-70).

• Brutus is flawed in his judgments.  In Act I, scene ii, hearing the shouting of the crowd and flourish of trumpets, he tells Cassius, "I do fear the people/Choose Caesar for their king." At Sardis, when he and Cassius quarrel, critics have suggested that Brutus's judgment, clouded by the love he has for Cassius, demonstrates his inability to distinguish his own motives from his noble principles.  Ironically, Brutus's remark about the "tide in the affairs of men" (IV, iii) becomes tragically true for him.
• His moral judgment is flawed.  Often he confuses his own motives in his idealism and his devotion to the principle of republicism.  In his soliloquy of act II, Brutus deceives himself into believing that he is solely concerned about the public good: 

Th'abuse of greaness is when it disjoins/Remorse from power; and, to speak truth of Caesar,/I have not known when his affections swayed/More than his reason.  But 'tis a common proof that lowliness is young ambition's ladder,/Whereto the climber upward turns his face;/But when he once attains the upmost round,/He then unto the ladder turns his back,/...scorning the base degrees...(II,i, 18-27)   

• Later Brutus becomes guilty of what he suspects in Caesar.  In his quarrel with Cassius at Sardis, Brutus is guilty of this very pride, especially when he manipulates his troops, telling them they can leave if they do not believe in his "noble" causes. 

Truly, as Marc Antony declared after the assassination of his beloved Caesar, in Brutus, as in the other conspirators,

O, judgment, thou art fled to brutish beasts (is it not interesting brutish looks much like Brutus?)/And men have lost their reason!...(III,ii,105-106) 

In the play, Brutus decides to join the conspirators against Caesar because Cassius convinces him that it's for the good of Rome. Brutus loves Caesar, but loves his country more.  It's the same reason many families were split during the civil war; they loved one another, but they had a higher sense of loyalty and responsibility to their homeland.  Love of country is responsible for may heroic and heinous acts all throughout history.

However noble his reasoning for turning against Caesar, one fact remains, though:  they were friends.  Any way you slice it, it still ends up being a strong case of betrayal.  They were not best friends by anymeans, but they were friends nonetheless.  Friendship inherently entails some sort of loyalty, and Brutus clearly forgets this as he joins a group of men who are set against the leader of Rome for a variety of reasons, most of which are the furthest thing from noble.