Mercutio says Tybalt is a very adept duelist.
Mercutio is very aware of Tybalt’s reputation. He has a temper and likes to fight. He is also very good at it. Mercutio calls him “prince of cats.” At the Capulet’s ball, Tybalt was ready to fight Romeo just for being there. Capulet tells him he has a temper.
Later, Mercutio tells Benvolio that Tybalt is an expert with the sword.
More than prince of cats, I can tell you. O, he is
the courageous captain of compliments. He fights as
you sing prick-song, keeps time, distance, and
proportion; rests me his minim rest, one, two, and
the third in your bosom: the very butcher of a silk
button, a duellist, a duelist … (Act 2, Scene 4)
This foreshadows the fight that Mercuito and Tybalt will have. Tybalt’s quarrel is not with Mercutio, but with Romeo. However, as we already know, Romeo is not the fighting type. He is in love with Juliet and considers Tybalt family. Mercutio will protect Romeo at all costs, and he also knows that the boy will not protect himself. Also, Mercutio is not one to back down from a fight.
What wouldst thou have with me?
Good king of cats, nothing but one of your nine
lives; that I mean to make bold withal, and as you
shall use me hereafter, drybeat the rest of the
eight. … (Act 3, Scene 1)
When Mercutio and Tybalt fight, Romeo tries to intervene. He gets in the way, trying to stop the fight. Ironically, that is what causes Tybalt to kill Mercutio. This is why Mercutio curses both of them as he dies. He is upset at Romeo for being so foolish as to get in the way of what should have been an ordinary duel between two expert swordsman. Romeo, grieving over Mercutio’s death and his part in it, fights and kills Tybalt to avenge Mercutio's death.