Hell, the Animorphs series could get very frank at times with the themes of war and the moral implications of a guerilla war on children... There was a point in my reading where the ploy of having one of the heroes lose a limb (in an animal morph) was kind of something the series had desensitized me too. In the 13th Book, they have an alien essentially scalp himself to prove a point that he was not under control of the brain slug antagonists of the series... and it does get worse. And this was a series for an intended audience of late elementry school to early middle school aged kids.
The dirty little secret about YA novels is you can get away with a lot of graphic violence depictions to a much younger audience than TV or Movies or Video games and this is doubly so for books targeted to a male audience. This is because of two factors: YA has an over-surplus of novels with a female audience in mind compared to the novels for a male audience, and no matter what gender is reading, most parents are so happy to see the kids reading that they don't bother to check if they are reading a kosher amount of violence.
Additionally, when writing novels with no visual elements, people will visiualize the story differently. A kid who enjoys violence and gore will probably paint his mental picture red with blood, where as a kid with a lesser appreciation can self censor. Also, Kids are really capable of handling complex situations that adults didn't think they could.
For example, I personally remember watching an episode of "Batman Beyond" in middle school that dealt with a fairly obvious Steroid metaphor. Now, as a kid, I used to privately praise the episode for this one scene where the heroes' mother finds the drug paraphernalia in his school bag and there is a heated argument and I distinctly recall the mom expressly using the word "Drugs" during this scene, which most cartoons would dance around. When I went off to college, I binged watched the show out of a sense of nostalgia and to my great surprise, the word "Drugs" was never used in that scene (instead the ridiculous name "Slappers" was the term for the drug... it was a nicotine patch like delivery system). That was something I had added because, yeah, it was drugs... that's what the episode was discussing without saying the "D-Word". And I checked because the series was censored when parents got wind of some things that were much darker but that episode wasn't censored. And with that said, they did show rather unabashed results of prolonged drug use and what is effectively an on screen OD. All of which I had remembered from the time of viewing and found my adult self rather taken aback by the boldness of the episode to be so direct and so subtle about the topic.
One fun thing you should do (since you clearly like Japanese Media) is watch a few episodes of Super Sentai and it's cross-Pacific American Counter-part and prepared for a culture shock. Most Americans would balk at some of the shows more serious episodes for being that dark. But the funny thing is, the Japanese viewer will watch U.S. Power Rangers and be equally horrified (Japan gets away with the serious elements by very zany humor... it's usually a live action cartoon when they want to be funny, and this offsets the humor. The American version largely avoids the mood whiplash and the lack of the more wacky humor leaves the Japanese asking the same "This is meant for children?" response to Power Rangers that Americans have to Super Sentai).
