I was very puzzled by the doctor. First I was surprised that a psychiatrist would go out in search of a patient. Doctors don't generally go to such lengths. If a potentially dangerous patient goes missing, wouldn't you call the police? Or was he a doctor with a military facility? Wouldn't he have a couple of MPs with him, in that case? This doctor showing up alone (or technically brought by Goss) seemed to lend believability to Peter's claim that the military was experimenting on him.
Second that the doctor would smoke drugs, although I agree he may have been trying to gain Agnes's trust. Still, he's alone, trying to nab a dangerous patient. Wouldn't he want all his wits about him?
Third that he would promise to help Agnes find Lloyd. (And how did he know about Lloyd, unless after RC and Goss tipped him off to Peter's location, he did some research about Agnes.) If he did actually know something, that seems wickedly unethical. To get her hopes up, tease her along that he knows where Lloyd is and whether he's okay -- that just seems so low, when all he wants is to take Peter back to a mental facility.
And finally that a psychiatrist would be so naive as to get close up with a patient that has so clearly gone off the rails. He should have known better. But I've noticed in lots of movies that psychiatrists often think they can "talk him down" with a crazy patient -- with the syringe of sedative at the ready. It's hard to believe a real doctor would take that chance.
So was he real? Was he with the military, come to retrieve their 'research project'? Was he not a doctor but someone who knew Peter in some other way? Or was he really a psychiatrist from wherever Peter had been kept, who was trying to find Peter and help him, who saw the paraphernalia and knew Agnes could be manipulated, who underestimated how dangerous Peter had become?
I guess we can't know, which is frustrating.