The Best Explanation: If Jason is at the bottom of this mystery, the most logical conclusion would trace back to the N.A.T. Club. In the earlier seasons of the show, we learn that Ali blackmails the members of the club (Jason, Ian Thomas, and Garrett Reynolds) and threatens to expose them for the perverted videos they have hidden away. And maybe it starts that way. But with the deaths of Garrett and Ian, perhaps Jason's mission has shifted. Maybe he just wants revenge. Maybe he wants the liars to pay for all the damage they've caused over the last decade.
In reality, that could just be the tip of the iceberg. Maybe Jason has somehow connected himself to Mary Drake. Maybe Jason knows about Charlotte and wants to avenge her. There are so many ways to weave Jason back into everything that you just can't ignore him as a suspect.
The Most Damning Evidence: The best way to pin it on Jason is simply to remember he's a DiLaurentis. This means he's directly related to Charlotte DiLaurentis, to his mother Jessica, to Mary Drake, and to the Hastings family. He's kind of at the center of everything. Jason has appeared in the seventh season of the show (at least the first half, anyway), so it's safe to say he's still in the game and still in and around Rosewood. We know he's involved in the Carissimi Group (which also has ties to Charlotte), and we know he's been conspicuously absent in the latest crop of episodes. This gives him room to execute the revenge plot to end all revenge plots.
