Romeo is banished in Act 3 because he kills Tybalt, who is Juliet's cousin. The fight between Romeo and Tybalt occurs shortly after Romeo and Juliet are married in secret. Mercutio, Romeo's friend, is killed by Tybalt, and in a fit of rage and grief, Romeo retaliates and kills Tybalt. The Prince of Verona, who has warned both families previously about their ongoing feuds, decides to exile Romeo as punishment for taking the law into his own hands and for causing further bloodshed. In the eyes of the law, Romeo's actions are seen as an act of revenge rather than self-defense, and thus he is banished from Verona.
