this is a ridiculous question.
Tybalt was killed by Romeo, who was angry at Tybalt for killing Mercutio.
Tybalt is her cousin. Romeo is her husband. Her mother and her nurse are angry at Romeo for killing Tybalt. Should she be angry also or stick by her man?
Your question is a bit vague, but at the Capulet's house near the beginning of the play, Romeo (a Montague) comes to their party. Tybalt is engraged by his presense and later seeks revenge on Romeo by attacking him and provoking him to fight. Mercutio (Romeo's friend) then starts to fight Tybalt so he will stop attacking Romeo and Mercutio is stabbed by Tybalt. This then makes Romeo angry and Romeo ends up killing Tybalt for revenge.
Tybalt is Juliet's hot-headed cousin. He kills Romeo's best friend, Mercutio, in a feud. Romeo, angry and wanting revenge, kills Tybalt.
When you say "mad" you mean "angry" not "crazy", right? Romeo experiences extreme anger in Act III Scene 1 when his friend Mercutio is killed in a swordfight with Tybalt. Romeo is especially angry because not only is his friend dead but Romeo feels that it is his fault. Mercutio blames him for causing his death by trying to break up the fight. Romeo's anger is even more at himself than at Tybalt, but he directs his self-anger to Tybalt, losing all control.
Tybalt was asking for a fight with Romeo. Since he was married to Juliet and Tybalt was now his cousin, he refused to fight him. Then Mercutio asked to fight Tybalt and in a way kind of tempted him to fighting. Tybalt ended up killing Mercutio because Romeo got in the way when trying to stop the fight between the two. So now Romeo is angry that his friend Mercutio is dead and Tybalt doesn't have a scratch. So, Tybalt and Romeo fight and Romeo kills Tybalt.
In Romeo and Juliet, the prince banishes Romeo from Verona to the neighboring land of Mantua. The prince banishes Romeo because he kills Tybalt (Juliet's cousin) out of his rage for Tybalt killing his friend Mercutio in a brawl. Romeo is not sentenced to death by the prince, because he killed Tybalt only to avenge his friend's death. Also, the prince only exiles Romeo, so that there is a fair sentence that can appease both the Capulets and Montagues.
Mercutio accepts Tybalt's challenge on Romeo's behalf. But it may be debated whether Tybalt would have murdered Romeo had Mercutio not intervened--probably not. The only point where Tybalt was actually on the point of killing Romeo was when Romeo attacked him, and Romeo was the one who prevented Tybalt from killing him.
You just answered your own question. "Why does romeo get banished after killing Tyblat?"
For killing Tybalt
With the killing of Romeo
Romeo's friend and cousin is Benvolio. He is known for being calm, level-headed, and a peacekeeper in the play "Romeo and Juliet."