I reread them about ten years ago. They always had teen themes, but the later books definitely got dark. They held up really well, honestly.

The others mentioned it, but yeah, they were 'serious' from the beginning. The very first book had a character being ripped in half and eaten semi-alive. Applegate was just good at somehow pulling you along and not thinking about how much body horror was front and center.

You had 'slugs' crawling through people's ear canals as they are (sometimes) violently held underwater, tigers (and such) ripping through (and I'm trying to remember the exact quote but can't, so paraphrasing) 'soft bodied worms' (the taxxons, I think they were called?), the hork-bjir (I'm surely misspelling that) with knives/daggers sticking out of all parts of their bodies that would use them to slice and dice... at one point I think there was the threat of a character having to change while trapped in a small bottle, which would cause them to horrifically be squished... a literal 'torture beam' that would cause pain...

and likely a bunch more that I've forgotten.

The second series the author wrote was similarly horrific, though obviously in different ways. I kinda don't remember that one as much. There might have been ravines of bleeding flesh, shoulders being pulled out of sockets by being hung with manacles on the wrists and such, or I could be mixing it with something else.

So, yeah. The author definitely had some ideas for horror if ever there was a desire to not be semi-action oriented.

The ending mostly felt forced, because the writing style combined with the setting would have taken another hundred books to even get close to finish. If I remember correctly, the entire series took place within the span of a year or so, with not super serious attention paid to timing. The enemy faction was spread out across the entire galaxy, or something like that, so it mostly just became a very quick 'and they lost/won' with some mystery, and then the author threw a big 'fuck you' out there by ending it with a black screen action sequence (imagine I Am Legend's cinematic ending) right as the mystery came back to the characters a while in the future.

Fun Edit: I actually think I got my interest in BDSM started with the first animorph book I read (#7, I think? Or one of the 'not in numbered order' numbered order ones) when the villain of the day turns out to be very good at constricting/binding/choking.

They ended up having torture, genocide, murder, slavery and wrestling with the ethics of all of that. I’m still amazed they did all that in the early 2000s

Dunno, I read that like 20 years ago. But from my vague memories I don’t think it felt different than reading detective books like The Three Investigators.