To me, the real question behind "what happened?" is a personal question. A question of our individual beliefs.
From early on in the show this theme starts to show itself. Dr. Tenma believes in the equality of human life; that every life has value, and that beyond that, every life has equal value. No one's better or more important than anyone else.
Remember the Turkish woman screaming at Tenma in the beginning, because he saved another person that the higher-ups assigned him to, even though this lady's husband arrived at the hospital first?
This is starkly contrasted with Nina being so angry at Tenma very early on in the show for saving Johan. Just like the Turkish woman was screaming at Tenma, "Why didn't you save him?!", Nina screams at Tenma, "Why did you save him?!"
Gina's done some excellent work with the translation of "Another Monster." Reading through that should help to clear up some of the confusion about the plot. I highly recommend checking that out
But the main point seems to be this overarching theme of the value of human life.
What is the value of human life, really? Are all people equal?
This series has a lot more torque when you can really feel this idea on a visceral level. If you look into it deep enough, you'll see a simple question. But the way it's portrayed and the scenarios it brings up, and the way it executes them (no pun intended...>_<) is what makes it art.
Johan says that we're all equal only in death. I find this really disturbing. And I find it disturbing because it means something to me personally -- and as with most art, I think that's one of the goals here. To resonate with people and to get them, ultimately, to use the art as a medium to look into themselves and reflect on their own lives.
I don't think Tenma had multiple personalities for this reason. I don't think that would be relevant to the theme and it would have no reason to play into the plot. Johan is very clearly depicted to be a real person, and the perpetrator of these crimes. Tenma's beliefs about the equality of human life and his reluctance to kill are central to the driving force behind the theme (which would be the interplay between the idea that "all life is equal" and the idea that "we're all only equal in death."
In other words, I don't think it would be necessary for Tenma to have multiple personalities to contribute anything relevant to the themes of Monster, and for that reason I think we can take Tenma at face value and not have to think about it much beyond that. He's a doctor that has extreme conflicts because of his belief in the equality of human life, while feeling like it's his responsibility to end someone's life at the same time. This conflict is a thematic driving force behind the whole series.
There're a lot of other things to get confused about. There were some sketchy things as far as the plot goes, and some things were really confusing, but it all seems a lot less confusing when you keep the main ideas in mind. The themes behind Monster are much easier to follow and understand than the nuances of the plot, and similarly, I think keeping the themes in mind is a great way to understand the plot better.
As I said before, I think one of the main goals of the series is to appeal to people personally -- to get people to reflect and introspect. And this really magnifies the torque and gravity of the whole series. You feel things on a whole different level when it really hits you on a deep personal level -- and it can, and it will, when you put yourself in Tenma's position (for example), and let the questions Johan asks Tenma be like Johan asking YOU those questions.
You believe in the equality of human life? That everyone's life has equal value?
Having been struggling with being suicidal since I was 13 or so, for example, the scene with Milosh (when he's ready to commit suicide by jumping off the bridge at the Red Light District) really hit me hard.
Grimmer of course, especially emphatically in that scene, represents the power of love and compassion to transcend the impossible. As far as the series reveals, he's the only person who came out of 511 Kinderheim alive who also maintained a level of humanity.
And that's putting it lightly. I wonder why so many people like Grimmer so much. The friend of mine who recommend Monster to me said (way before I got to the episode where Grimmer first appears) that even though the whole show is awesome, it'd be worth watching just for Grimmer.
I don't think it's a coincidence. What I see in Grimmer is the ability of the human spirit to transcend logic. I think Grimmer's role in the series would best be described as a parallel to the 'Starfish Story':
One day a man was walking along the beach when he noticed
a boy picking something up and gently throwing it into the ocean.
Approaching the boy, he asked, What are you doing?
The youth replied, Throwing starfish back into the ocean.
The surf is up and the tide is going out. If I don't throw them back, they'll die.
Son, the man said, don't you realize there are miles and miles of beach and hundreds of starfish?
You can't make a difference!
After listening politely, the boy bent down, picked up another starfish,
and threw it back into the surf. Then, smiling at the man, he said
I made a difference for that one.
The times I've most wanted to commit suicide have been the times I felt intellectually trapped. Basically, it was like being trapped in the mindset that Johan provoked Milosh into. In my own life, I had no friends left; I had been suicidal, and the last person I'd talked to at the time, the only friend I had left, told me that I should just do it, but be sure not to hurt anyone else.
'Monster' was especially powerful to me considering my experience with feeling completely abandoned and neglected. Many times I've really believed what Johan told Milosh - that birth is nothing special, that it's just a meaningless flash in the universe, that you don't have to be wanted to be born -- there's nothing special about being born, about being alive.
When you believe deep down that your life has no value, and you don't believe that love has any importance, and that this is just 'survival of the fittest' -- that's when it seems easiest to follow through on trying to commit suicide. The world just seems like a bleak and loveless place. I've been told before to 'do the world a favor and kill myself,' and have read multiple things promoting the belief that no one will care if you die; that your life doesn't mean anything special, you're the only person who can care about you, etc.
And again, that's why I think Grimmer is such an awesome character to so many people. That one scene with Milosh was so powerful. Ready to commit suicide -- he had no mother, and after talking with Johan, he was now convinced that his life had no purpose. His little miserable journey through the Red Light District is a perfect parallel to so many people in the world who don't experience any love at all, and everyone they meet is abusive, neglectful, or just don't show any concern about their existence.
It's no surprise he's ready to commit suicide. Anyone in that position would be.
The philosophical and intellectual quandaries we can get ourselves into are worth struggling with. They really are. Questioning your beliefs is good -- if they can't even stand up to your OWN questioning, how could they stand up to someone else's or survive in the real world? You'll crumble if your beliefs aren't backed up by more than just intellect. Intellect can very easily be defeated if your emotions don't line up with it.
Monster has so much more to offer than that, but to me, that's my favorite theme from the show - the one portrayed by Grimmer. He was severely traumatized but somehow managed to stay human. And he shows that even in a world that's completely upside down and where nothing seems like it could be helped, that simply being human can be a light in the darkness.
Why did Milosh's life have value? Did it have intrinsic value? Or would it not have mattered if he committed suicide?
These questions are all like the remarks of the man in the Starfish Story talking about how there're too many starfish to save. Grimmer's like the little boy that's throwing as many as he can back into the ocean. Milosh was like one of those starfish. In that one scene where Grimmer saves his life, all those philosophical questions are put to a stop. I think the whole point of scenes like that is to basically put the viewer in a position where they can't do much else but get emotional about it.
So the contrast is really evocative. Intellect and emotion, philosophy and love. You could really think about these themes your whole life and never arrive at a single conclusion.
And that's my take on the ending. Who knows what happened to Johan? Did he die? Did he go on to commit more murders? Did he go off to see his sister or mother? Did he go off alone?
We'll never really know. "Another Monster" is confusing to me too! (thanks again Gina, by the way - great work)
But it works because of that. We can never really know, just like in real life, we can never really know. The timelessness factor here is due to the fact that the series leaves questions that can never be answered -- just like real life poses us questions that can never be answered.
I think that's a big part of what makes this series so great. You're going to have complexities in every series, just because life is so complex, and every series depicts some aspect of life. But Monster deliberately implies all these themes and implements them into the series to a very, very large degree.
And it leaves us on our own to make our own judgments about our beliefs and to really question the nature of things. It calls us to look at how flimsy and amorphous our beliefs can really be, and especially brings to light how we can't understand why the world works the way it does, and how it's impossible to evaluate all the consequences of our actions. Good intentions can lead to bad things -- like Tenma saving Johna. Did that mean that all life isn't equal or that his belief in life being equal was wrong? There's no final say on this.
And there's no final say on what happened to Johan at the end. Tenma saves him once again. What are the results of this? Who knows. It's left a mystery to emphasize the fact that there's no way to know what the results of our actions or beliefs will be. We believe in what we believe in and try our best -- the full consequences of that are beyond the scope of human vision.
That said, I do think the importance of love is emphasized in Monster. I don't think it's COMPLETELY up in the air. I think this is pretty clearly presented by Kinderheim 511 completely messing up all the kids that went there, Franz Bonaparta's remorse and change of heart, Grimmer (yes, Grimmer is awesome enough that just the fact that he exists proves a universal truth), Nina forgiving Johan (regardless of how much Johan cares about being forgiven - and I think that's an important point that he didn't even seem to care or be surprised that she forgave him, because a lot of times in life, showing love and support to people will NOT give you good results you expected, and a lot of times can CHALLENGE your belief in love and valuing human life, instead of reinforcing or supporting it)...etc. etc.
...and etc. etc. The examples go on and on. I could keep talking about this but I think I've presented the idea clearly enough by now. As clearly as I can with my tendency to jump around from topic to topic, anyway.