The streets of Verona are not safe when both the Capulets and the Montagues are out and about in Shakespeare's Romeo and Juliet. The situation intensifies quickly as Romeo, Mercutio, and Benvolio, all Montagues, sneak into a Capulet feast one night. One of the Capulets, Tybalt, Juliet's cousin, recognizes Romeo and is furious that an enemy would dare to enter the Capulet home, much less look at Juliet in such a way.
Of course, Romeo does a lot more than just look at Juliet. By the end of the feast, they are in love, and before much longer, they are secretly married. In act 3, scene 1, the day after the wedding, Mercutio and Benvolio meet Tybalt and his friends on the street, and hostilities quickly escalate. Romeo soon enters, and Tybalt calls him a villain. Romeo does not want to fight Tybalt now that they are kinsmen by marriage (although Tybalt certainly doesn't know that), so Romeo refuses to draw his sword.
Mercutio, however, has no qualms about that. If Romeo will not fight Tybalt, Mercutio declares that he will. Romeo tries his best to separate them, but Tybalt is too quick, and he stabs Mercutio. Mercutio falls, and after a few more witty comments (he just can't help himself even though he is mortally wounded), he dies. Romeo is horrified and guilt-stricken. If he had stepped up to fight Tybalt, Mercutio might still be alive. Tybalt has fled the scene but makes the mistake of returning, and Romeo kills him.