One of the major benefits I got from re-watching Monster is that I now understand the plot & themes way, way, WAY better. It makes me cringe thinking about my prior misconceptions about this series. BUT NOW, MY FRIENDS. NOW I HAVE ALL THE ANSWERS. I'M LIKE THE MONSTER MESSIAH.
WARNING: If you haven't watched all the way up to the last episode of Monster, go watch the series and then come back. In the meantime, SPOILERS EVERYWHERE. RUN WHILE YOU STILL CAN!!!!
I was prompted to write this after reading some people's post-series impressions of Monster. When criticizing the characters, I've read many people say Johan had "no motive" and that he was the arbitrary "absolute bad" to counter Tenma's "absolute good." But there IS a method to his madness and a reason why he became the way he is, and it can be uncovered by answering one question: why did Johan grow up to be an indisputably bad person, whereas Nina grew up to be a fairly normal, successful young woman? Moreover, this question helps in answering THE driving question of the series: why do people become 'monsters'? Do all people have equal potential to become monsters (are all lives equal?), or are certain individuals much more susceptible to it than others?
By exploring this question through its huge cast of characters, some answers begin to emerge. For instance, this analysis discusses the circumstances that led Grimmer and Roberto to become wildly different adults despite both being Kinderheim 511 kids. The primary difference between the two is how they react to stressors. Roberto reacts with aggression and is likely easier to provoke, whereas Grimmer is more passive by nature and only lashes out in self-defense. This small difference possibly made Roberto easier to manipulate and control. I believe the difference between Johan and Nina is likely something small as well. Namely, Johan demonstrates a much greater inclination to internalize his traumas. While Nina actively tries to repress traumatic memories (the healthier response), Johan allows those traumas to shape his identity.
Of course, this characteristic alone is not enough to create a Johan. Rather, as with all characters in Monster, traumatic events act as 'triggers' that exploit what ever monster-forming potential lies within them. There were specific events, some unique to Johan, that prompted his monster to grow. Conversely, despite the traumatic events in Anna's childhood, there were specific events unique to Anna that helped her to contain her monster.
1: twin girls
I've always found it... odd that the twins' mother decided to dress them up as two girls. It's much more socially acceptable to force a girl to have short hair and wear shorts than it is to force a boy to wear a wig and dress. But the cross dressing wasn't the real problem; what really had a profound effect on Johan was that he was forced to be his sister. People didn't see them as two twin girls, but as one girl. That Johan still cross dresses as Nina shouldn't be written off as some hokey plot twist; it shows just how fucked Johan's self-identity is. He means it when he says, "I am her and she is me."
2: unwanted child
Urasawa was wise to reveal Johan and Anna's back story in reverse chronological order. The earlier the memory, the deeper the effect it had on them, and Urasawa saves the earliest memory for the very last scene. It's not clear why Johan was so convinced that his mother meant to give him away. This is pure extrapolation on my part, but perhaps dressing as his sister led him to make a logical leap: "If she dresses me like my sister, that must mean she prefers her." What is clear is that his mother's decision serves as the basis for the philosophy Johan acts upon throughout the course of the series: that there is no inherent value in his life and that human life in general is worth very little. When Johan asks that orphan boy in Prague, "Who wanted you? What is your reason for living?" he's largely projecting his own issues onto that poor kid.
3: the red rose mansion
So we know that Johan wasn't actually there. But as mentioned earlier, Johan makes a habit of internalizing trauma and using it to form his identity. Moreover, since he sees himself as Anna and vice versa, it's very... Johan of him to believe that their experiences are shared too. Though this event hugely traumatized Anna, it probably had just as big an impact on Johan. It's implied that, when recounting her experiences to Johan, Anna likely neglected to mention the most important thing: Bonaparta telling her she mustn't become a monster. Even if she didn't dwell on that memory, it certainly had some subconscious effect on her. Anna was probably better able to retain her sanity thanks to Bonaparta's message, whereas Johan is left thinking that the world is one crazy ass place and that human life is indeed expendable. It's this line of thinking that encourages him to commit his first murders soon afterwards.
4: the nameless monster
General fucking Wolf. Who knows why he thought it was a good idea to give Johan the same name as the monster in the book. The effects of this name-gifting are exacerbated by the fact that Johan had just seen the "scenery at the end of the world" (the wasteland) for the first time, and so he could completely understand and relate to the monster in the ending pages of the book. Even prior to meeting General Wolf, Johan had already developed a strong attachment to the book. The psychological effects Bonaparta's books had on kids were covered extensively in the series. Since Johan internalizes absolutely everything, he's affected worst of all. This book basically served as the blueprint for his life. When later hopping from foster parent to foster parent, he would try out a new name each time, then eventually "consume" his parents and discard the name, just like the monster in the book. Johan later states that his goal is to be the "last man standing at the end of the world"; he doesn't mean a literal apocalypse, he just wants to meet the same end as the nameless monster. Notice that whenever someone saves Johan's life (i.e. General Wolf and Tenma), he repays them by allowing them to see the same "scenery" he did when he was on the brink of death. The book doesn't simply parallel Johan's life; he modeled his life after it.
5: kinderheim 511
I think the effect Kinderheim had on Johan was greater than the characters made it out to be. Johan was by no means a normal kid when going into Kinderheim, but... the way I see it, they took in a budding amateur and turned him into a professional. The staff at Kinderheim likely upped Johan's aggression and allowed him to hone his abilities at manipulating people. The one thing that's certain is, Kinderheim turned Johan's memories into a jumbled mess. Johan's self-identity was already fucked up before Kinderheim, and I imagine it only got worse after his memories were tampered with.
And all of this leads to the Johan we see by the start of the series. Even though he's forgotten the nameless monster book by this point, he still subconsciously bases his life around it and can even recite whole passages of the text. He destroys all of the people who helped in creating him and burns down all of the places related to his childhood (i.e. The Three Frogs, the Red Rose Mansion, Kinderheim 511, even the Munich library where he found and remembered the nameless monster book). His ultimate goal is for all this to lead to his eventual self-annihilation. But it's only once he fully rediscovers his past that he decides to end his life. While in Ruhenheim, Grimmer once said Kinderheim 511 kids were "fictional characters" (in a non-meta way). This describes Johan better than anyone. Thanks to Kinderheim tampering with Johan's memories, he needed to get slapped in the face with the truth a couple times before he even realized it himself. Nina telling Johan the truth about the Red Rose Mansion was probably when he realized, not only was his life modeled after a children's book, but even his own memories were falsified. If he's not a "real person," then what is he?
So how does Tenma fit into all this? More than anyone else, Tenma gave Johan a reason to keep living. Johan fully intended to let Anna kill him that night in the Liebert household. But unlike General Wolf, Tenma saved his life not out of self-interest or charity, but in order to uphold a sacred belief. Tenma's beliefs and actions directly contradict everything Johan had believed up until that point. Not only is human life sacred to Tenma, but he asserts that Johan's life IS valuable, just as much as anyone else's. I think Johan was grateful Tenma found his life valuable enough to save and even risk his career for. I also think Johan killed the hospital directors as a genuine show of gratitude, a "you'll accept this gift whether you like it or not!" kind of thing. But at the same time, I doubt Johan bought Tenma's "all life is equal" schtick even for a second. After all, Johan heard Tenma essentially say that self-interested, immoral people "deserve to die"--I'm sure even as a kid, Johan understood the irony of the situation. Johan was sure that, if he showed Tenma the monster he revived, Tenma would turn his back on his silly belief and prove once and for all that human life IS unequal. Johan thought Tenma would decide he was "unneeded," just as he suspected his mother did when he was a child.
Ultimately, Johan was wrong about Tenma. I think once Tenma knew the circumstances of Johan's childhood, he and Nina both decided that to kill Johan for being a monster would be to blame the result (Johan) rather than the factors that led to the result (everything written above). Of course, this isn't perfect justification. I myself don't hold much sympathy for Johan; it may not be his fault that he's a terrible person, but he IS still a terrible person. Though considering Tenma spends the whole series saving the lives of criminals, it's not too big a stretch for him to end up doing the same for Johan. (And besides, Tenma is absolute good.)
To return to one of the questions posed at the beginning: do all people have equal potential to become monsters? Strictly speaking, I think the answer is no. Even if Nina and Johan had both heard Bonaparta's warning and had both went to the 'normal' orphanage, I think Johan would have still been at greater risk for having a negative life outcome. But in the end, that some people can more easily become monsters than others isn't really the point. It was by no means "inevitable" that Johan would turn out the way he did. If the twins' parents had successfully gotten away from the scary eugenics people, Johan and Anna would've in all likeliness become normal adults. Feelings of rejection, loneliness, rage, self-loathing, fear: these are the things that turn people into monsters. Johan is an extreme case, yes, but his contempt for humanity didn't come from thin air. He wasn't born a devil, but his circumstances encouraged him to become one. For final clarification, I do not intend to argue that Johan was purely a victim of his environment. I do think there was some agency on Johan's part, as I doubt many other kids would be able to will themselves to kill as quickly as Johan did. But it seems that many people watch Monster and conclude that Johan was destined to become evil, when that almost certainly was not the case.