The turning point in Romeo and Juliet, according to Harold Goddard's The Meaning of Shakespeare (), is the point at which Romeo is unable to free himself from his violent upbringing, to turn his back on the incorrect teachings of history, his family and the feud - and "give all to love" (in the words of Emerson). When Mercutio and Tybalt begin to tangle, instead of sticking to his plan to befriend Tybalt (a Capulet, to whom he's now related by marriage), he instead tries to fight violence with violence, or as Goddard puts it: "He descends from the level of love to the level of violence and attempts to part the fighters with his sword". Here's the specific passage:

Draw, Benvolio, beat down their weapons

Gentleman, for shame, forbear this outrage!

Tybalt, Mercutio, the prince expressly hath

Forbidden bandying in Verona streets:

Hold, Tybalt! good Mercutio!

There is an argument to be made that at this point Romeo either could have let them fight it out (which perhaps would not have resulted in a death), or he could have revealed everything, including his secret marriage to Juliet. Instead he tried to interpose with the use of force, with the results we know, including the pathetic moment when Romeo answers Mercutio's reasonable question, "Why the devil came you between us? I was hurt under your arm", with: "I thought all for the best." Mercutio then turns away from him in disgust, anger, disbelief (take your pick) and says: "Help me into some house, Benvolio, or I shall faint."