What are the similarities and differences between Mercutio and Tybalt?Romeo and Juliet by William Shakespeare

In Shakespeare's Romeo and Juliet,
Tybalt is a flat character, who consistently exhibits a nature that is belligerent,
resentful, and obstinate.  His first appearance in the play is in the opening fight
scene in which he challenges Benvolio:

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What, art thou drawn among these heartless
hinds?

Turn thee, Benvolio, look upon thy death.
(I,i,65-66)

Death remains on
his mind with regard to the Montagues throughout the play.  When Tybalt espies Romeo in
the Capulet house he wishes to kill him immediately, but is restrained by his uncle,
Lord Capulet, who calls him a "princox" (I,v,91)

Even the
hot-tempered character, Tybalt is ready to dual when he encounters Mercutio in Act III. 
On such a hot day in Verona, unfortunately, Mercutio, too, is angry with the "Prince of
Cats" as he has named Tybalt.  And, ironically, it is Mercutio who challenges Tybalt. 
But, unlike Mercutio, Tybalt is not above taking unfair advantage of Mercutio as he
reaches around the intervening Romeo and slays
Mercutio.

Like his name, Mercutio has a changeable nature. 
Whereas he has been bawdy and jovial in the two previous acts, Mercutio displays much
cholera in the third act, showing eagerness to fight a Capulet, and in this way he is
much like Tybalt.  At his end, his bitterness overcomes his loyalty as he
dies:

A plague
be o' both your houses
(III,i,99)

Before the third
act, Mercutio seems much unlike Tybalt.  He is playful, tossing bawdy jokes at the
nurse, urging Romeo to reveal himself in Act II with such quips, and entertaining his
friends with his eloquent monologues.  He is the center of attention whenever he is
around unlike Tybalt who sulks in the background.  Nevertheless, the two characters both
seem too extreme to be able to last without tragedy.