After Tybalt kills Mercutio in a duel, Romeo is enraged by the death of his best friend and kills Tybalt. Romeo runs away and is banished from Verona forever. This incident begins a whole new series of events that ultimately result in the Romeo and Juliet's double suicides.
Both Tybalt and Mercutio die in the town square. It is the day of Romeo and Juliet's marriage, Benvolio and Mercutio are walking along when they encounter Tybalt with a group of his flunkies. Soon after Romeo enters, excited about the news that he is now married to his beloved Juliet. Tybalt challenges him to a duel. Romeo does his best to avoid the fight and Mercutio, sickened by Romeo's weakness, accepts Tybalt's challenge. Romeo pleads with the two not to fight and in the midst of his efforts, Tybalt stabs Mercutio and Romeo in turn, slays Tybalt.
Tybalt and Mercutio were both killed in the town square. It is on the day of Romeo and Juliet's wedding. Mercutio doesn't think Romeo will fight Tybalt, so he wants to fight for him. He believes his friend is too much of a lover, rather than a fighter. Romeo comes on to the scene feeling happy and excited that he is now married to his love. He has no idea what is waiting for him. Mercutio and Tybalt start dueling and Romeo tries to get between them to stop the fighting. Tybalt then stabs Mercutio under Romeo's arm, leading to Romeo's best friend's death. When Romeo realizes his friend his dead, his rage takes control of him.
"Alive, in triumph! And Mercutio slain! Away to heaven, respective lenity, and fire-eyed fury be my conduct now! Now, Tybalt, take the villain back again, That late thou gavest me, for Mercutio's soul is but a a little way above our heads, staying for thine to keep him company: Either thou, or I, or both, must go with him."
Romeo lets his fury and rage take over and he kills Tybalt. This is what gets Romeo banished. This is the beginning of the tragedy that is to play out. Romeo has let his temper get the best of him, and it will cost him everything that matters to him.
Tybalt died in the streets of Verona after a fight with Romeo (stagings usually depict that Tybalt is killed in a sword fight, but Shakespeare's directions simply read, "They fight. Tybalt falls."), who sought revenge because Tybalt killed Romeo's friend Mercutio. Tybalt previously challenged Romeo over what Tybalt perceived to be an insult when he overheard Romeo at Lord Capulet's party.
In Act III, Scene 1, Tybalt and the Capulet men come upon Mercutio and Benvolio in the street and ask about Romeo. Mercutio, who obviously doesn't like Tybalt, becomes difficult and avoids the question. Benvolio, afraid of a fight, warns the men to get out of the streets because the Prince decreed that men caught fighting would be put to death. Romeo soon enters the scene and, when insulted by Tybalt, backs down, arguing Tybalt doesn't really know him. Romeo even claims he loves Tybalt, having just married Juliet, Tybalt's cousin:
Tybalt, the reason that I have to love thee
Doth much excuse the appertaining rage
To such a greeting. Villain am I none.
Therefore farewell. I see thou knowest me not.
Tybalt dies outside the great hall in Verona. he dies because he had killed Romeo's friend Mercutio this enrages Romeo so he kills Tybalt. that is the best answer i can give to you.