What is a theme in Romeo and Juliet act 3 explained?

One of the most prominent themes in "Romeo and Juliet" is
the destruction that comes from a long standing hatred, in this case, between two
families.

Though the hatred between the Capulets and
Montagues is made known at the outset of the play, it is not until Scene 3 that the
seriousness of the feud is truly displayed.  The fight in this scene starts in much the
same way that the fight in scene 1 starts: with petty taunting and the exchange of
insults.  In fact, Tybalt is enraged by Romeo's newfound kindness.  This result of his
marriage to Juliet is misinterpreted by Tybalt as patronizing and ironic cruelty. 
Mercutio, not even a member of one of the families, with his quick wit and
confrontational nature, encourages the fight that Romeo attempts to
prevent.

This fight results in nothing but destruction. 
There are the immediate deaths of both Mercutio and Tybalt, which leads to Romeo running
away, which leads to Juliet's faked suicide, Romeo's suicide, and Juliet's real
suicide.  There is also a skirmish in the meantime which kills
Paris.

For a fight that started as something very petty
(and almost silly) many serious consequences result.  The emotions and irrationality
shown by the characters in Act 3 are the culmination one of Shakespeare's main points of
the entire play, which is to say that even the pettiest of conflicts, when properly
fueled, can result in serious consequences.