...It has to be the judge. He was taking justice into his own hands by killing all these people who he thought were murderers. He'd done that before. And... I guess he must have shot himself afterwards, to make his death real? Maybe to avoid facing justice himself.

I'm still not sure why the doctor would help him fake his death, but... these people were scared and paranoid. He didn't actually kill all of them, some of them killed themselves or each other. It's not out of the question he could have been threatened or convinced that it was a good idea.