How is Brutus inconsistent in his Stoic philosophy?

Stoicism is a school of Greek philosophy which became hugely influential in ancient Rome, especially among the upper classes. Stoics believed that virtue was the highest good and that its cultivation was the only source of true knowledge. In practical terms, this involved the acceptance or rejection of external goods, objects, and pleasures insofar as they did or did not conduce to virtue. Stoics became famous for their remarkable ability to withstand great physical pain, hardship, and adversity. Their concentration on the overriding importance of virtue meant that they could develop some degree of immunity to the kind of destructive emotions that affect most people at some point in their lives.
Brutus in Julius Caesar considers himself a Stoic. Given the dangers of Roman political life, with its sudden reversals of fortune, confiscations of property and the ever-present prospect of brutal torture and even more brutal execution, it's understandable that Brutus would subscribe to such a philosophy. In act 2, scene 1 Brutus's wife, Portia, begs him to divulge his plans, even going so far as to cut her thigh to prove her devotion. Yet Brutus, ever the Stoic, shows no emotion whatsoever. Later on in the play, in act 4, scene 3, Brutus shows a similar lack of emotion when discussing Portia's death.
Brutus's Stoicism goes some way towards explaining his motivation for killing Caesar. Brutus's knowledge of virtue gives him knowledge of Caesar's true intentions regarding his political ambitions; Caesar is measured up against Brutus's standard of virtue and found wanting. He gives the impression that his decision to join the conspiracy against his friend is entirely cool, rational, and calculated. No hint of emotion is involved, supposedly.
But Brutus cannot maintain his Stoicism indefinitely. The love for his friend finally prevails, causing him to regret his part in the assassination. Brutus also realizes that, unlike a true Stoic, he would not be able to endure the implacable wrath of Octavian. Nevertheless, Brutus, in committing suicide, does still maintain an admirably Stoic attitude:

Caesar, now be still. I killed not thee with half so good a will (Act V Scene v).

In other words, when it comes to killing Caesar, Brutus hesitates somewhat; some degree of emotion for his friend still gets in the way. But when it comes to the act of killing himself, Brutus has no hesitation. So he goes to his death as a Stoic, albeit an inconsistent one.