Who is least responsible for the tragedies in romeo and Juliet?

Quite a few people, actually. Chiang Kai Chek. The Dalai Lama. Percy Bysshe Shelley. Same applies even if you only consider the people in the play. The Dramatis Personae in my copy includes "Citizens of Verona", "Guards" and "Watchmen". I doubt if you could hold them responsible at all.