If the Capulets and Montagues disturb the peace again, the Prince will kill them.
The Prince's ultimatum to the sparring families is the following lethal one:
"If ever you disturb our streets again,
Your lives shall pay the forfeit of the peace."
According to this stamentent, a death is requered in order to be dramatically satisfying, and it fatalistically suggests one of two paths for the families; peace is reached, or lives are lost.