Animorphs was a seriously gruesome series of books. There is a point semi early on where one of the characters was infiltrating a guarded compound and ended up fighting a hork-bajir1 as a silverback gorilla. It ended up slicing open his midsection and left for dead. The author described in vivid detail the character holding his own intestines, trying to push them back in, all while trying to not blackout from the pain and change back to human (which would remove the damage). As a kid that fucked me up and I had to stop reading for a couple hours.
Oh, another instance is one of the characters getting sprayed by roach spray and was slowly, painfully dying as the nerve agent slowly turned off their internal organs.
Both of those were pretty fucked up situations and the books are filled with them and many more great, definitely not for kids moments.
One of the main characters actually comes from a very abusive family and as a coping mechanism he'd often change into a red-tailed hawk as the ability to fly allowed him a freedom he never had before. He ended up getting stuck like that because if you stay as a morph for too long, it becomes permanent. Through the books he starts having to fight against the instincts of being a hawk, fight to keep his humanity.
God, I could just gush about the books, but sadly I never got to finish them. I should reread them now.
Sorry for how long this is.
1 basically a species of plant eaters who's bodies features razor sharp spikes (to enable climbing redwood like trees on their home planet and as a means of defense) but were entirely enslaved and weaponized by the main enemy, the yeerk. The yeerk are a species of very small, slug-like creatures who can completely control almost any creature with a brain as long as there is an easy way to get to it. Turns out ears are a great access point and a lot of races have them.