A true friend is a weapon and a shield. A false friend is a deadly poison in the cup.
Julius Caesar had his share of both. There was the friend that would stab him in the back... literally. And then there was the friend who would start a war to avenge his death.
Through Brutus and the conspirators, Shakespeare represents friends who were not loyal to any but themselves. Through Antony and Octavian, he shows loyalty that lasts until and after death.