In each of Gus' visions of the killer, he is wearing a mask with a long beak. This is significant for two reasons. One: We know that the killer utilizes a purple powder, synthesized using alchemy, to knock out his victims before he kills them, and we are able to recreate that powder at one point in the game. Two: in the days of the black plague, Plague Doctors would wear beaked masks filled with herbs to protect themselves from miasma in the air. Ergo, we can conclude that the killer filled the mask's beak with a different substance to protect himself from the effects of the purple powder.