In Romeo and Juliet, it's important to understand that Tybalt was asking for a fight with romeo. Romeo, since he was married to Juliet and Tybalt was now his cousin 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.