Answer:
The punishment fit the crime.
Explanation:
After a fight in which Tybalt gets into a street fight with Mercutio and ends up killing him, Romeo stabs him to death.
The one who started the fight was Tybalt, while Romeo always wanted to keep the peace.
The Prince, because of this situation, makes the decision to exile Romeo. The decision was fair, because Romeo always wanted to stop the fight, and it was Tybalt who killed his friend first.
This lets us see that the Prince is a compassionate and logical character, but that he still enforces justice.