The Rogers (servants) because I thought the murderer had to arrive early to prepare.
Dr Armstrong because it would be convenient for him, and there was nothing written from his point of view.
I figured that someone "dead" was not really dead somehow, but couldn't think of how. The only one I thought of was Mrs Rogers.
I figured the judge's death was strange: not only he was killed when everyone but him rushed away from the room, but someone also bothered to put a wig and stuff on him - I didn't think it made sense. But it did not occur that he could be conspiring with the doctor.
Next time I read a mystery novel like this, I should do it very slowly, and actually try to solve it myself.