I don't think it's Quinn, his performance telling Estes to leave Brody alone was too convincing. If Quinn wanted to take down Brody he would have done it.
Saul did just get an instant promotion and he got to take out Estes at the same time but I don't peg him for a mass murderer.
My biggest question is why did Brody and Carrie leave the memorial service when they did? Awful convenient. Could still be Brody or wait for it...... Carrie.
For a second I wondered if it was Mike that did it somehow.
My money is on Max, the tech guy from the van that always pals around with Virgil.