Friar Laurence had to banish Romeo instead of sentencing him to death although Romeo felt not being with Juliet was living death.
This was because Romeo stood down to a fight against Tybalt so Mercutio steps in and fights Tybalt and Mercutio gets stabbed and dies, in revenge Romeo kills Tybalt. Romeo did not think about what would happen if he killed Tybalt, he was very rash and it cost him by not being able to be with Juliet and being banished in tern.