These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: The manga is dedicated to exploring whether there is such a thing as Pure Evil, or if anybody, no matter how heinous, can be redeemed. The series' conclusion is that while anybody can be redeemed, it is titular character Johan Liebert's refusal to seek redemption that makes him a monster. A sadistic chessmaster and manipulative bastard since he learned to walk and talk, Johan is on a personal journey to become, in his own words, as dark as possible, all while trying to find out if there is anyone as evil as he is out there, or failing that, force the heroic Dr. Tenma, and/or his own pacifistic sister, Nina Fortner, to kill him in cold blood. In this way he hopes to establish that even the best people can become just like him. Along the way he convinces children to jump from rooftops, gets a recovering alcoholic drunk before throwing him off a building, and convinces various serial killers to murder every foster family he has ever lived with, as part of his quest to become an unperson. Numerous Freudian Excuses are proposed for Johan's behaviour, yet each and every one is ultimately undermined, revealing that in the end, Johan simply is what he is, and what he is, is pure, unadulterated evil.
Al Green's "Let's Stay Together" being played in Episode 23 when Roberto and Eva are at the bar.
"Medium Shot" with its jittery violin and clattering percussion (including the triangle!) is the one of the most delightfully creepy songs in the OST.
Ear Worm: How much of an ear worm is the Ronette's "Be My Baby"? People have reported getting it stuck in their head from reading about it in the manga.
Ending Fatigue: 74 episodes. Shots sustained simply to reproduce the manga rather than narrative purpose. Repeatedly winding up suspense to yet another lack of climax. Monster in general is a series that likes to take its sweet time in doing things. In general, it loved to do this thing where it would basically make the main protagonist, Tenma, disappear for a little while, introduce a side character or small set of side characters, give them A Day in the Limelight and sufficient Character Development to get the audience to like or remember them to some extent, and then much later reintroduce Tenma to clean up whatever the new characters were doing. The epitome of this would be the Bayern arc, for introducing about six new characters that went on with their own problems for, in the manga, about 15-20 chapters before Tenma even shows back up, and even then, the main plot is largely disconnected from this. All-in-all, the characters this arc focused on really didn't impact the plot in any huge way but was largely still compelling enough to read through to when it would.
Family-Unfriendly Aesop: Franz Bonaparta's books contain plenty of these like, "Sometimes you're simply screwed no matter which choice you take."
Genius Bonus: Aside from the extra understanding gleaned by those with medical degrees, there's a bonus for film or animation students. Lipsky could be a puppeteer just because it's a part of his character, or because it's both creepy and sad...or it could double as a Shout-Out to the Prague school of animation, which, based on when Monster is set, would have still been going strong during Lipsky's childhood. This particular localized trend produced a lot of work based on puppetry and dark fairy tales, both elements which compliment Monster nicely. And of course, Lipsky's first met in Prague.
Like You Would Really Do It: Will Tenma actually confront the problems of his Thou Shall Not Kill code? Of course not, don't be ridiculous. Possibly subverted by the finale, however. If Johan escaped to continue his rampage, it wouldn't do Tenma's idealism any good.
Johan skips right past it in the first volume and keeps going from there.
One could even say that this series doesn't have one. People are redeemed (sometimes at a large cost) for crimes so heinous even death wouldn't be enough in most series. At the end, even Johan is forgiven by his sister, who says "Even if we were the only two people left in the whole world, I would forgive you."
One of the main themes of the series is the question of whether or not the Horizon even exists: is there a point where a person is so far gone as to be truly irredeemable?
In the end, the answer is a mixture of yes and no. No one, not even Johan is truly beyond redemption, but what makes Johan irredeemable is the fact that he doesn't want to be redeemed.
Even this is ambiguous as Johan's lack of desire for redemption seems to stem from his belief that he is irredeemable. "Some things can never be amended."