In Julius Caesar, how do the plebians (people) betrayal of Brutus help develop the plot?

In Act III, Scene 1 of Julius Caesar,
Brutus promises Marc Antony that when he goes to the pulpit to provide the reasons for
Caesar's death, he will tell the Romans that Antony "speaks by leave and permission" to
them afterward.  The implications of this statement of Brutus, that Antony is
essentially in accord with the conspirators, certainly conflict with the purposes of
Antony to turn the crowd against the conspirators.  So, when the Romans hear him, they
become more and more perplexed that Brutus should give his permission for Antony to
stand before them and make insinuations against the honorable intentions of the
conspirators. Then, when Antony produces the proof of Caesar's love of the people,
Caesar's will which gives each citizen money--a will Antony says he will not read
because it will inflame them--the Romans begin to truly question the motives of Brutus
and the others.  Further, Antony's rhetoric stirs the crowd emotionally so much that one
plebian shouts "They were traitors!" and another, "They were villains, murderers!" 
Finally, all shout,

readability="5">

Revenge!  About! Seek! Burn! Fire! Kill! Slay!
Let not a traitor live!
(3.2.215)

At this point, the
crowd becomes an irrational mob, and Antony's prophecy of his soliloquy over Caesar's
body becomes true,

readability="7">

A curse shall light upon the limbs of men;

Domestic fury and fierce civil strife
Shall cumber all the parts
of Italy;
Blood and destruction shall be so in
use,(3.1.282-285)

The civil
war that thus ensues after the plebians turn against Brutus and the conspirators, of
course, creates the sequence of events that compose Acts IV and V as Antony joins with
Lepidus and Octavius Caesar, the adopted son of Julius Caesar to battle with Brutus and
Cassius's armies in order to restore the republic ruled by the Caesars and rid Rome of
all those who have conspired against Julius
Caesar.