Why did Tybalt kill Mercutio?I'm typing a paper about the villain in the play and I think it's Tybalt because if he wouldn't have killed Mercutio,...

I think it is important to remember that Tybalt's
complaint is with Romeo and not with Mercutio. Remember that Tybalt was upset with
Romeo's presence at the Capulet part; it was Mercutio who actually had a legitimate
invitation.

In fact, when Tybalt confronts Mercutio (Act
III, scene i) it is only to determine the whereabouts of Romeo. When Romeo appears,
Tybalt stands down against Mercutio with the line: "Well, peace be with you,
sir: here comes my man."

It is Mercutio, then,
who continues to provoke Tybalt, particularly because Romeo refuses to fight. Romeo then
comes between Mercutio and Tybalt in hopes of separating them. It is at this moment that
Mercutio receives his fatal wound. It is not intentional on the part of Tybalt; it
happens only because Romeo is in the way.

It could be
argued that Romeo is as much to blame for Mercutio's death as is
Tybalt.