Portia is dead by her own hand. She's swallowed coals, a most painful — and some would say, fitting — way of death. By her suicide she takes on the sins of the men and attempts to expiate them; that is, in the manner of her suicide she, in metaphorical terms, internalizes the painful, rash, hot decisions that have brought the state to civil unrest. But in doing so, she does not contain and remove the difficulties facing Rome. She is ineffective, for this is not a play about what a woman could do, but a play about men and men's affairs.
The news of her death to Brutus is delayed. For the first one hundred and forty-six lines of the scene, the reader is unaware that Portia's death is probably the underlying motivation for Brutus' passionate quarrel with Cassius. What is Shakespeare's purpose in delaying such news? Impact. The sudden realization of what has happened gives Cassius and the audience a sudden insight into Brutus: the action of the scene and its real motivations and the change in Brutus' and Cassius' friendship. Moments of impact such as these offer a pause, a catching of breath that reveals multitudes.
Note that the love that Brutus felt for Portia is transferred to the male, non-sexual sphere in his friendship with Cassius. Loss and betrayal are essential elements of grief, but Brutus, unable to speak these disloyal thoughts against his wife, transfers his feelings to Cassius. It is Cassius who has betrayed him. It is Cassius who leaves him.
