All of them had a role in all of the events. But in my opinion, I would have to say Mark Zuckerberg, Sean Parker, and Eduardo Saverin. The brothers just sued them because they were the first to come up with idea. But it was Zuckerberg, Parker, and Saverin who were directly involved in the issues.
Zuckerberg and Saverin caused distrust in each other and, with Parker, were the ones who managed "The Facebook". They were the ones who made it such a success (causing the lawsuit since everyone wants money). Everything was too connected to each other that so it's hard to pinpoint someone and say who was at fault.
