Romeo and Juliet
The feud between the Montagues and the Capulet
Mob violence can be contagious. It is a well-known psychological phenomenon. When large groups of people get out of control people feel much safer joining them than resisting them. This would be the same thing in Verona streets. The city becomes a powder keg. This is why the Prince comes down so harshly on any citizen that again disturbs the peace.