How does the death of Mercutio initate a definitive tum toward tragedy in the play?

Romeo goes through a transformation of sorts. He snaps out of his love trance, at least for awhile, and becomes a young man full of vengeance for his fallen friend. The slaying of Mercutio leads to the slaying of Tybalt and the chain reaction to tragedy is set.

Transformations of sorts? What sort of sorts?