Why does the prince not excute Romeo immediately and instead gives him a chance to flee in Romeo and Juliet?

Although Romeo's killing of Tybalt in a street brawl would normally make his life forfeit, the fact that Tybalt himself had forfeited his life by killing Mercutio in a street brawl, and that Romeo was therefore only carrying out the sentence of the law, was taken in mitigation of his sentence, which was reduced from the death penalty to banishment.