When Romeo kills Tybalt, how was that a turning point in the play?
Tybalt's death is a turning point for several key reasons. First, it forces Juliet to make a decision. Faced with the death of her cousin at the hands of her husband, she must finally choose between Romeo and her family. After a brief moment of recrimination for Romeo, she shows once and for all that she has turned her back on the Capulets to embrace her husband:
My husband lives, that Tybalt would have slain;
And Tybalt's dead, that would have slain my husband.
All this is comfort; wherefore weep I then?
But Juliet weeps because Romeo has been banished for the crime of killing Tybalt. This is the event that sets the final events of the play in motion. To allow Juliet to avoid marrying Paris, the Friar arranges for the plot to put Juliet into a death-like state until Romeo can return to take her temporarily out of the city. This ultimately leads to their deaths. The death of Tybalt is one of the crucial turning points in the play.