The devil is to blame. Really.
When Tybalt and Mercutio fight, romeo trys to defend Mercutio but ends up having him killed. Mercutio dies from Tybalt bringing the sword from under Romeo and stabbing Mercutio.
Tybalt was asking for a fight with Romeo. Since he was married to Juliet and Tybalt was now his cousin, he refused to fight him. Then Mercutio asked to fight Tybalt and in a way kind of tempted him to fighting. Tybalt ended up killing Mercutio because Romeo got in the way when trying to stop the fight between the two. So now Romeo is angry that his friend Mercutio is dead and Tybalt doesn't have a scratch. So, Tybalt and Romeo fight and Romeo kills Tybalt.
earlier in the play, romeo crashed a Capulet party. angry, Tybalt swore revenge with a sword fight. Romeo's friend mercutio ends up fighting Tybalt, and romeo attempts to intervene by stepping between them. Tybalt stabs mercutio, despite Romeos attempts to end the fight, and mercutio dies. Romeo and Tybalt then fight, and romeo kills Tybalt, and romeo is banished from Vienna. Merutio, romeo and Tybalt sword fight.
Mercutio is the one who decides to fight Tybalt.
It was when Tybalt killed Mercutio and Mercutio blamed Romeo for it.
In the street.
Tybalt is angry because Romeo and his friends (including Mercutio) crashed the Capulet party, and really angry because Lord Capulet wouldn't allow Tybalt to throw them out of the party. Tybalt tries to get Romeo to swordfight with him, but Romeo (who is by now secretly married to Tybalt's cousin Juliet) refuses and tries to make peace with Tybalt. Mercutio is enraged that his cousin Romeo won't fight, and says that if Romeo won't fight Tybalt then he will. Tybalt and Mercutio begin swordfighting. Romeo, still trying to achieve a peaceful solution, jumps between the two and tries to push Mercutio safely away from Tybalt. This makes it impossible for Mercutio to use his sword to defend himself. Tybalt, seeing Mercutio's chest exposed under Romeo's upraised arm, fatally stabs the defenseless Mercutio.
Romeo refuses to fight Tybalt. Mercutio steps in for Romeo and fights Tybalt. Romeo steps between them, trying to stop the fight. Tybalt stabs Mercutio. Mercutio curses both the Capulets and Montagues. Mercutio dies. Romeo is enraged and decides to fight Tybalt. Romeo kills Tybalt, then he runs. Prince Escalus comes to the scene. He announces Romeo's banishment.
if you mean fight, then Tybalt after Tybalt had killed mercutio
Mercutio starts the fight that ends with Romeo fleeing the city.
Mercutio forces the fight on Tybalt. Tybalt is really only interested in Romeo, but Romeo is not disposed to quarrel with his wife's cousin. Mercutio takes up the quarrel on Romeo's behalf for no reason other than that he is quarrelsome. Mercutio and Tybalt fought because Tybalt had challenged Romeo to a fight but Mercutio knew that Romeo was not strong enough to fight Tybalt so Mercutio took the burden of fighting Tybalt and eventually dies when he is stabbed by Tybalt under Romeo's arm. Romeo had ran out to block the fight and Mercutio got stabbed. Tybalt leaves, and then returns clearly angry. He is then slain by Romeo and the Prince, Montague, and Capulet and Lady Capulet come out. Romeo exits, and Romeo is banished.
They fight in the street, which is illegal, of course.