Tybalt dies because Romeo kills him.
Romeo kills Tybalt because Tybalt has killed Malvolio. The two of them were fighting and Romeo was trying to break it up. When he did, he got in Malvolio's way and that allowed Tybalt to kill Malvolio. So Romeo gets mad and kills Tybalt.
The consequence is that Romeo gets sent out of the city (he is exiled). But if you think about it, the real consequence is that Romeo himself dies. When he gets exiled, Friar Lawrence has to come up with his big scheme to prevent Juliet from marrying Paris. If he hadn't had to do that, Romeo wouldn't have thought Juliet was dead and wouldn't have killed himself.
In the play 'Romeo and Juliet' by William Shakespeare, Romeo and Tybalt belong to different 'gangs.' Each gang is loyal to a different 'house' or important town family. Tybalt belongs to the same dynastic family as Romeo's beloved new girlfriend,Juliet - he is her cousin. So after they marry, Tybalt is now 'related' to Romeo by marriage (a kinsman) but doesn't know it. So now Romeo doesn't want to kill him, or fight him even. Tybalt is surprised, puzzled and humilated in front of his followers by this new attitude and gets nastier and things escalate. In the muddle, Romeo's friend gets killed when he tries to get between them, and Tybalt gets stabbed in the melee. Because Romeo is exiled, the true letter/news about Juliet gets held up and he hears the news about her her 'death' first and impulively races off.