Why is Tybalt to blame for the deaths of Romeo and Juliet?
In Act I, Scene 1, when Romeo comes on the scene after the street fight which opens the play, he makes the comment that, while the feud between the Montagues and Capulets is based in hatred, it has more to do with a love for fighting:
O me! What fray was here?
Yet tell me not, for I have heard it all.
Here’s much to do with hate, but more with love.
Tybalt is the embodiment of the hatred that has long existed between the Capulets and the Montagues. Even Capulet himself has to restrain the hot-blooded Tybalt when the young man recognizes Romeo at the Capulet party (where Romeo and Juliet first meet). This encounter causes Tybalt to bear an insatiable grudge against Romeo, which leads him to the fight that, it could be argued, leads directly to the deaths of the two lovers.
In the first scene of Act III, Tybalt encounters first Mercutio and Benvolio, and then Romeo, in Verona. The Capulet has been looking for Romeo in order to gain satisfaction for the insult he gave him by attending the Capulet festivities in secret. He challenges Romeo to a duel, calling him a "villain." When Romeo, having (unbeknownst to everyone involved) just married Juliet, refuses to fight, Mercutio jumps in, decrying Romeo's "vile submission," and Tybalt kills him, in part because Romeo restrained Benvolio while they were fighting. When Tybalt returns to the scene, Romeo kills him in a duel.
Tybalt's death, which resulted from his challenge to Romeo, sets in motion a chain of events that culminates with the deaths of Romeo and Juliet. The Prince banishes Romeo from the city, and Juliet and the Friar come up with a plan to avoid her scheduled marriage to Paris. The plot does not proceed as planned, though, and the two lovers wind up committing suicide. Because it was the death of Tybalt, caused in no small part by his recklessness, that drove Romeo from Verona, it certainly could be argued that he bears responsibility for the tragic end of the young couple. Certainly Tybalt is as captive to the hatred between the families as any of the other characters, but his hatred is particularly virulent and destructive.