Romeo and Juliet: Why did Tybalt want to kill Romeo?

Romeo had Lupus.

When you look at Tybalt's reactions to his encounters with Romeo and other Montague followers. He acts as protector and "chief swash-buckler" for the Capulets and he appears to feel that the Montagues represent a great threat to the Capulets. The first time we meet Tybalt is in Act 1, Scene 1, line 66. He enters with his sword drawn and tells Benvolio, both friend and cousin of Romeo, to "turn thee...Look upon thy death." When Benvolio says he has no need to fight and wants to keep the peace, Tybalt responds with, "...talk of peace? I hate the word as I hate hell, all Montagues and thee." This further displays Tybalt's disdain for the Montague clan. We meet Tybalt again in Act 1, Scene 5, at the party the Capulets are throwing. Tybalt spots Romeo at the gathering and is enraged by Romeo's actions. He states, "What dares the slave come hither, covered with an antic face, to fleer and scorn at our solemnity? Now by stock and honor of my kin, to strike him dead I hold it not a sin." (Act 1, Scene 5, 56-60) In this Tybalt is past the point of wanting to just protect the Capulets, he wants to kill Romeo. Even Capulet himself is not that upset by the fact that Romeo is at the party. He states on line 66 of the same scene, "Content thee, gentle coz, let him alone...And, to say truth, Verona brags of him To be a virtuous and well governed youth. I would not for the wealth of all this town Here in my house do him disparagement." Even these comments by Capulet do not lessen Tybalt's hatred of Romeo. Before Tybalt exits from the scene it can be seen that he is planning something against Romeo when he states, "But this intrusion shall, now seeming sweet, convert to bitterest gall." (Act 1, Scene 5, 92-93) Tybalt is not planning to forget that Romeo intruded into the Capulet's ball and he will get revenge. Vengeful is a good adjective to describe Tybalt. When we meet him again in Act 3, Scene 1, we will see just how vengeful he can be. Tybalt and his companions happen upon Mercutio, and Benvolio. Tybalt asks to have a word with Mercutio, then Mercutio, almost mockingly, converses with Tybalt until Romeo enters the scene. Tybalt immediately drops his conversation with Mercutio and Benvolio saying, "...Here comes my man..."(Act 3, Scene1, 55) Tybalt calls Romeo a villain, to which Romeo replies, "...Tybalt, the reason that I have to love thee Doth much excuse the appertaining rage To such a greeting. Villain am I none."(Act 3, Scene 1, 61-63) Tybalt does not want to hear any of this from Romeo and he is obviously agitated by Romeo's reasoning. He tells Romeo that his new found reasoning does not excuse him for any of his wrong doings and orders him to turn and draw. Romeo protests saying that he never has injured Tybalt and insists that he could never fight against a Capulet, a name so dear to him.

When Mercutio falls, seriously wounded, Tybalt exits. But he comes back in time for Romeo to challenge him to a duel to avenge Mercutio's death. Romeo kills Tybalt.