In Romeo and Juliet, Act 3 scene 1, Tybalt dies, who kills him? And what are the killer's consequences?

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.