How does Romeo interfering with Mercutio and Tybalt's fight affect the outcome of Romeo and Juliet?
If he hadn't have interfered, the play, most likely, would have had a much better ending.
If Romeo hadn't have thrown out his arm in between Mercutio and Tybalt, Tybalt would not have miscalculated his rapier and wouldn't have stabbed Mercutio. That means that Mercutio wouldn't've died, which means that Tybalt would have lived as well, because Romeo had no reason to kill him. Which means that Romeo would not be banished. Which means that Juliet does not fake kill herself. Which means that Romeo does not ACTUALLY kill himself. You see my point?
The strings of fate in Romeo and Juliet are very fragile; it's like dominos. You knock one over, they all fall.