Tybalt sees Benvolio and Mercutio out on the street and he wants to fight them. Romeo comes up to them and Tybalt wants to fight him. He has been wanting to fight Romeo ever since the party at the Capulets'.
Romeo has already married Juliet at this point (secretly) so he feels that he and Tybalt are relatives and shouldn't fight. As Romeo tries to break up the fight between Tybalt and Mercutio, Tybalt sneakily stabs Mercutio.
Romeo gets mad because of this and kills Tybalt.
Romeo killed Tybalt ( Juliet's cousin ) because Tybalt ended up killing Mercutio ( Romeo's best friend ) first . In the beginning , Romeo came up to both Tybalt and Mercutio to stop the fight because he was married to Juliet already ( so basically you can't kill your wife's family ) . But of course that didn't work , which ended up with Mercutio's death and later on Tybalt's .
Tybalt is Juliet’s cousin (Which makes Tybalt a loyal Capulet) While Romeo is a Montague (A sworn enemy to the Capulet’s)
Romeo is pressured to go to the Capulet’s ball and is seen there by Tybalt who asks Mr. Capulet if they can throw Romeo out and fight him. Lord Capulet says no, that Romeo is a respectable man and that he can say.
Tybalt is still furious at this, and, two days later, Tybalt sees Benvolio and Mercutio (Benvolio’s foil) Benvolio is a calm man and Mercutio, is well, not. When Tybalt sees Benvolio and Mercutio, he approaches them looking for Romeo Montague. When Romeo shows up, an in-law to the Montagues now, he sees Tybalt and approaches, Tybalt insults Romeo in front of everybody and Romeo shrugs it off since he is married to Juliet now (But only Friar Lawrence, Nurse and I suppose Romeo’s servant Balthasar know other then the husband and wife)
When Romeo walks away, Mercutio steps up to fight Tybalt, Romeo tries to stop the fight since Mercutio is Romeo’s best friend and now Tybalt and Romeo are now Cousin-in-law. But, Tybalt, being a professional sword-fighter, he stabs Mercutio when it is very un-expected.
Benvolio takes Mercutio away after Mercutio died, while Mercutio was dying however, he foreshadowed: ‘A Plague on both your houses’ Romeo cries and gets angry at Tybalt for killing his best friend. If Romeo controlled his temper, Tybalt would have been in trouble, but, Romeo fights Tybalt and kills him, now Prince will have to kill Romeo, Romeo runs to Friar Lawrence to hide. Thus the climax starts to take off.
With Love and Care,