Unmarked spoilers follow.
Anyone remotely familiar with Johan's memetic personality has a fairly good idea of this character's limits (or lack thereof) for delicious diableries. He is a by-word for evil: he brings small children to red light districts, and watches his loyal followers die with utmost serenity; he murders his caregivers, one innocent elderly couple after another; he teaches children to play on roof ledges and trip up homeless people on the street; one of his life goals is to cheerfully corrupt the man who had saved him from certain death. He does not have a moral justification for his actions, and he does not care one iota about whether the person he is going to kill (or lead to suicide) is guilty of anything more than offering a helping hand to the wrong person. And yet...
It's not quite the thorough mental torture from his childhood, nor the overwhelming love he bears for his sister. It's not that his sense of identity is extremely volatile to begin with, only to be further weakened by subliminal and explicit brainwashing. It's not that his loyalty to his mother's revenge is, if not admirable, then certainly sympathetic. It's all of these together—and the recognition of a monster within—that make him equally as pitiable as he is horrific. He is a person guilty of so much—yet how can someone without a clear sense of self and with a morality so thoroughly perverted by external forces be held completely culpable? Johan is unambiguously a product of his environment; to which degree are his actions, then, his own?
- But that might be simplyfing things. We may assume that Johan is purely a product of his environment, but at the same time his sister went through exactly the same things he went through, and only he turned into such a homicidal maniac. Then there are the occasional hints of supernatural power and the implication that Johan is, in fact, The Antichrist. Certainly at no point do we see him "snap" and he seems like a perfectly normal, happy child until he and his sister are seperated from their mother...at which point he starts murdering and corrupting people a little too quickly to think that it was that which turned him in the titular monster. While trained from birth to be an amoral Ubermensch, why did it only work on him and not on Nina? And why did all attempts to replicate the experiments always produce "inferior" results—monsters, but none as monstrous as Johan. Perhaps his childhood did not create the evil within Johan, but merely unleashed and gave form to that which was there. The pity of Johan may be that he was doomed to be wicked all along, and indeed by the time of the main plot he is so evil that he has become bored with his own villainy.
There is a literary tradition of introducing in your characters traits that will unequivocally produce one type of response—for example, Satan in Paradise Lost is depicted as a hero for a large part of the text—only to show the reader how easily his or her own opinions and ethics can be manipulated through his or her emotional response. Johan functions like this: his monstrosity is built up (arguably even exaggerated) to create in the reader a feeling of total repugnance and terror. His later revelation as a deeply disturbed character who may not even be in conscious and willful command of half of his actions tests the reader's ability to sympathize with a character who was previously so completely reviled.
It could be argued that the horrific extent of Johan's and Bonaparta's evil is used as a device to gage the reader's threshold for sympathy and forgiveness. The acts each of these commit are utterly despicable, yet the series asks us whether these people can still be redeemed. Even if Bonaparta's atonement can never measure up to the degree of damage that he has caused, he is nonetheless credited with Wim's goodness and perseverance even in the face of great tribulations and anguish; in this light, would it be right to take revenge upon him for his past actions? Would it be right to take revenge upon anyone, period?
Analogously, it could seem that Tenma's goodness—although nuanced, complex, and ultimately precluding him from committing a wrong that would potentially save numerous lives—is a similar exercise in stretching the extremes of human emotion and reaction. However, while Johan's evil would be used to establish the volatility of audience response, and, more importantly, our limits for pity and compassion, Tenma's goodness is deployed in order to see how lasting, enduring, and uncompromising good is.
Would Tenma have shot Johan at the end?
Depending on your interpretation of his character, Tenma is either shown as becoming more elastic in his morals and concessions—having shot Roberto, Kristof, and threatened various people (with dubious amounts of intent behind the threats), or much more rigid, realizing with increasing conviction that killing someone, even a killer, would be the wrong thing to do.
Wim's father takes the final decision away from him is often reviled as a Contrived Coincidence, but as Wim's presence itself is contrived, this is a moot point. Tenma could have disabled Johan with a non-fatal wound. What is indisputable, however, is that Tenma chose to save Johan. Not for Nina's sake, not to save Wim's father, not to follow Runge, but because he deemed it the correct thing to do.
Monster and Fairy Tales
Even apart from the subplot featuring Bonaparta's story books, Monster dedicates a surprising amount of time to Fairy Tale Motifs and allusions. Casual references are strewn across the story: Eva references the Sleeping Beauty right in the first chapter, Nina's classmates tease her about her "Prince Charming," several chapters are named after fairy tales or their stock characters, Johan and Nina separately claim that they are from a "fairy tale town," Bonaparta sees himself as the Beast and Anna as the Beauty... and this is just the tip of the iceberg. This volume of references can hardly be incidental; but if they are intentional, what is their purpose?
The way most of the fairy-tale (as well as the biblical) allusions work within Monster is to give the characters—otherwise fully human—an archetypal dimension. Johan is not merely a victim of brainwashing nor a serial killer, but a monster equated with The Antichrist; Tenma is not merely a doctor with a heavy conscience and an extremely potent sense of right and wrong, but a messianic Hero and a Knight in Shining Armor; Bonaparta is not only a scientist in love with a woman who hates him, but an outright beast. In this way, Monster establishes itself as something greater and more universal than a crime story of a Japanese surgeon chasing a deranged youth across Germany—it becomes a story of archetypal good and evil without sacrificing any of its realism or originality.