Animorphs.
Cool friends fighting aliens...more accurately, the pariah of a fascist civilization, minutes before being eaten alive in front of said "ccol friends" persuades and then bioengineers human child soldiers to facilitate an end to an ill-conceived and failing war now reduced to unilaterally exterminating a parasitic, physically disabled species, itself undergoing a violent civil rights movement on their own planet based on their self-recognized flaws, struggling to realize its place in a universe where godlike beings exist and decide not to offer remedy(rules of the god game) and spectate while the parasites overwhelm all vulnerable species in the known universe.
The child soldiers agree to resist the parasites, but at least a minority of them believe genocide is the wrong answer. After being physically and emotionally tortured, shot, repeatedly disemboweled and having their limbs hacked or bitten off by hosts of the parasitic species, however, all of the child soldiers begin taking violent, morally devastating actions that end their lives as they know them.
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Always makes me shudder, when the gang is inside the ocean acquiring some monster.
Especially giant squid, Reading sphere at 10 years old or however old I was terrified me.
Pretty horrifying when they're all half drowning trying to demotph and remorph in the ocean.
