What does Romeo do in reaction to Mercutio's death?
Mercutio is Romeo's dearest and closest friend. Though he holds a more skeptical, much bawdier view of love than our hero, he's still fiercely loyal to his boon companion. He doesn't take Romeo's fondness for romantic love in the least bit seriously; nor, for that matter, does he have much time for the dominant code of masculine honor expressed in unseemly street brawls and duels. Nevertheless, Mercutio is as quick-tempered as anyone. He is certainly more of a fighter than a lover, the exact opposite of Romeo.
So when Tybalt challenges Romeo to a duel and Romeo refuses, Mercutio is quick to step into the breach and defend his honor. Sadly, in the ensuing duel in Act III Scene I, Mercutio is killed by Tybalt. His fatal wound may not be as deep as a well or as wide as a church door, but it's enough to kill him nonetheless.
Romeo is distraught at the death of his friend, not least because he was partly responsible for it. He stepped between Mercutio and Tybalt to stop the fight, but Tybalt took advantage of this and made a sneaky sword thrust which finished off Mercutio. Tybalt returns to the scene and he and Romeo fight. This time Romeo prevails and Tybalt lies dead. From now on, the tone of the whole play darkens considerably, and there is a tragic inevitability about what subsequently happens.