I believe that Johan is a metaphor for pure evil, nihilism, and nothingness. I think the fact that the show makes it seem like Tenma imagined Johan’s revelation in the hospital alludes to the illusoriness of Johan. The constant reference to Johan as a fiction also hints at this idea. Another clue is that the show doesn’t fully explain Johan’s motives. Sure, terrible stuff happened to him. Yet, Nina experienced similar traumas, and she turned out fine. Johan’s origin story doesn’t fully explain Johan’s malice. Johan doesn’t make sense; he doesn’t fit neatly into the real world.
A person is grounded in a particular socio-cultural background, in a particular time and space. Johan isn’t like that. He is nameless, enigmatic, and seemingly everywhere. He is more like a concept of evil that plagues the lives of everyone– serial killers, families, orphans, and doctors. If you look at the totality of the characters, there is nothing that binds them together other than that they were all human compared to Johan –he even makes serial killers look sane.
Johan’s murders are cold and emotionless as Lunge once described. There is no goal, no motivation. Johan, in this sense, resembles nothingness, which takes us away one by one indiscriminately –arbitrarily. We desperately try to make sense of it, ascribe intentionality or even a governing set of laws. But in the end, I think such efforts are futile, for I agree with Wittgenstein that “Death is not an event in life: we do not live to experience death.” Perhaps this is why it makes one’s blood run cold to see Johan in action; he does not make sense and the thought of a meaningless end terrifies us.