Romeo to a duel, but Mercutio steps in instead. Tybalt slays him, but seemingly by accident. Romeo pursues him, and kills Tybalt in a heated battle. This leads to Romeo's banishment.

All of these are instances in which Tybalt affects the tragic nature of the play. Yet let's not forget Juliet, her parents, and the Nurse. Tybalt's death affects each of them, driving the tragic plot further. For Juliet, it forces her to reconsider her love for Romeo. Her parents drive her into marriage with Paris, thinking it is her sadness over her cousin's death that keeps her so melancholy. And the Nurse turns her back on Romeo, seeing him only as a murderer and a disdainful man. Thus, Tybalt is quite influential throughout the play.

He is a nephew to Lord Capulet and a cousin to Juliet. He does not speak many lines, but he influences the entire course of the play to a degree that exceeds his seemingly minor role in it. Throughout the play, he demonstrates his angry, resentful, and stubborn nature. Tybalt urges on the fight of the Capulet and Montague households. In addition to his being belligerent and stubborn, Tybalt also has no qualms about fighting unfairly.

Tybalt's execution of Mercutio is pivotal to the tragic ending of Romeo and Juliet.

In Act III, Scene 1, a fiery Mercutio meets and argues with an equally explosive Tybalt, who encounters hostility from Mercutio immediately. For, when Tybalt says he will have a word with him, Mercutio provokes him by replying, "make it a word and a blow." The ensuing argument then becomes further inflamed by the arrival of Romeo, who in his euphoric happiness seeks to ameliorate things. He declares to Tybalt that he now loves him, but without providing a reason for this change of heart first. When an already riled Mercutio hears Romeo, he accuses his friend of "vile submission" and draws his sword. 

Tempers flare and aggressive action escalates. Still, Romeo tries to stop the others by reminding them of the Prince's having forbidden "bandying in Verona streets./Hold, Tybalt, good Mercutio!" But, Tybalt reaches with his sword under Romeo's arm and stabs Mercutio.

This murderous act against Mercutio by Tybalt enrages Romeo so much that he then slays Tybalt in retaliation. This sets in motion the circumstances and actions of both Romeo and Juliet that bring about their tragic ends:

• Because he has killed Tybalt, Romeo is banished.
• Believing that Juliet grieves so much over Tybalt's death, her parents decide that she should marry Paris. This decision leads Juliet to seek help from Friar Laurence, who gives her a potion which makes her seem dead in order to stall any actions by the Capulets, so that Romeo can come and "bear thee [her] hence to Mantua" (4.2.).
• Away from Verona in Mantua, Romeo is told mistakenly that Juliet has died; desperate, he purchases poison and rushes to the Capulet tomb. There he encounters Paris and slays him.
• Romeo finds Juliet, who he believes is dead. 
• Romeo drinks the poison.
• Juliet comes out of her drug-induced state and asks Friar Laurence where Romeo is because she remembers their plan. But Friar Laurence, who has just discovered Paris's and Romeo's bodies, becomes frightened as he hears the approach of the guards. He tells her that Romeo "there lies dead,/And Paris, too" (5.3). He offers to put her safely in a convent, but first "go, good Juliet, I dare no longer stay" (5.3). The nervous Friar Laurence flees.
• Left alone, Juliet goes over to Romeo's dead body. She hears the watchman. In despair, she snatches Romeo's dagger and kills herself.