Benvolio and Mercutio are confronted in the street by Tybalt as he is looking for Romeo to challenge him to a duel. Tybalt knows that Benvolio and Mercutio are always around Romeo so he asks where Romeo is "Mercutio, thou consortest with Romeo"(Act III.Scene i.Line 40). Then Mercutio and Tybalt start fighting then Romeo enters
As Mercutio and Tybalt are fighting Romeo enters and steps between them and tries to stop the fighting from happening. When Romeo is speaking Tybalt stabs Mercutio killing him "Draw Benvolio; beat down their weapons./Gentlemen, for shame! forbear this outrage!" (ACT III.Scene I.Lines 80-81). At this point Romeo and Tybalt start to argue and eventually get into a sword fight or duel themselves.