In Romeo and Juliet, another possible reason for the Prince's reluctance to follow through on the sentence of death that he has proclaimed for anyone who disturbs the peace of the streets of Verona is that he may reason that his putting of Romeo to death may ignite more feuding and simply perpetuate the hatred between the Montague and Capulet family. For, since he knows that already three civil brawls have erupted from only "an airy word" and caused Verona's senior citizens to take out their old weapons and resume their fighting, the Prince may feel that banishment for Romeo is preferable.
In addition, being the Prince, he may have been alerted that Friar Laurence has married Romeo and Juliet; thinking that their marriage may not yet be consummated, the Prince may want to be able to force the annulment of this marriage by keeping Romeo away long enough for such an annulment. In this way, too, he can avoid more feuding.
Or, he may have only been advised that Romeo was seen at the Capulet home on the night of the masked ball for Juliet. Then, the Prince may conjecture that if he banishes Romeo, Juliet's eyes will fall upon someone else, such as Paris--who is, after all, his kinsman--and she will not become interested in a Montague, the enemy of her family.
The Prince never offers us an explicit reason for only banishing Romeo and not having him put to death. However, we can infer that perhaps the Prince believed Romeo was justified in taking vengeance on Tybalt for the death of Mercutio. Mercutio was also the Prince's cousin.
Additionally, we also learn from Lord Capulet (of all people) that Romeo is known around Verona to be a fairly upstanding person with no record of violence in his past. It would make sense then, perhaps, for the Prince to spare the life of Romeo and only banish him.
We are not really told why, so this is one that we have to speculate on.
In my opinion, there are a couple of possible reasons why the prince has been merciful.
The first reason is that he might have heard about Romeo's real part in the fight. He might have heard Romeo tried to break it up and only got made when Tybalt killed Mercutio unfairly and unnecessarily.
Second, I imagine that the prince does not want to make the Montagues angry if he can help it. They are powerful, and it seems to me that he might want to try to keep them happy if at all possible.
First of all, the Prince has ordered death for any citizen who disturbs the streets of Verona, which Romeo, Mercutio, and Tybalt all have done. Technically, they all should be put to death. The Capulets bring Tybalt's dead body before the Prince just as the Montagues bring Mercutio's dead body, each family seeking vengeance for the death of their loved one.
The Prince questions Benvolio, since as his name implies, he is good and trustworthy (bene/bueno= good). Benvolio tells the Prince how Romeo greeted Tybalt pleasantly, refusing to argue and greeting him as a friend. At this point, the audience knows (dramatic irony) that Romeo is married to Juliet, making him cousin to Tybalt. We understand why Romeo won't fight Tybalt, but Tybalt doesn't understand. Tybalt still wants vengeance against Romeo for disrespecting the Capulets.
Mercutio, who claims that Romeo was no match to fight Tybalt, "The Prince of Cats," is enraged that Romeo will not at least defend himself. Because Romeo won't fight Tybalt, Mercutio decides to pick up the gauntlet. As a reputable swordsman, Mercutio would most likely defeat Tybalt easily. Unfortunately, Romeo tries to break up the fight, and while so doing, he ironically causes Mercutio to be fatally wounded.
Benvolio relates this entire story to the Prince, who sees that Tybalt was the instigator. It is clear that Romeo had tried on several occasions to avoid conflict. It was only after he saw Tybalt slay Mercutio that Romeo snaps in a brash, thoughtless, implusive moment in which he chases down Tybalt and kills him. Mercutio, cousin to the Prince, would (at least to some extent) be considered fairly avenged.
The Prince sees that a death sentence would be too severe under the circumstances and for a man of Romeo's character. Quickly, the Prince makes the decision to exile Romeo rather than put him to death. In fourteenth century Verona, exile was as close to death as one could get.
The Prince's decision to exile Romeo rather than execute him is due to the unusual circumstances of the crime as well as Romeo's exceptional character.