Tybalt has killed Mercutio, because Mercutio stood up for Romeo who didn't want to fight his new cousin. Romeo and Benvolio are in despair. "Ask for me tomorrow and you shall find me a grave man." 3.1.94-95
Romeo, now angry because Tybalt killed Mercutio, they fight and he kills Tybalt. After he runs away. "Alive, in triumph, and Mercutio slain. Away to heaven respective lenity, And fire-eyed fury be my conduct now! Now Tybalt, take the 'villain' back again That late thou gav'st me, for Mercutio's soul is but a little way above our heads, Staying for thine to keep him company. Either thou or i or both must go with him... tybalt slain! 3.1.118-130
The Prince, Montague, Capulet, and their wives come and see Tybalt dead. Benvolio explains to them what happens. The Capulets think that Romeo should die and the Montagues want him to live. The Prince banishes Romeo and sentences him to death if he comes back and fines the Montagues and the Capulets. "And for that offence Immediately we do exile him hence. ... But I'll amerce you with so strong a fine ... Let Romeo hence in haste, Else when he is ofund, that hour is his last." 3.1.186-195