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.
Use spoiler tags as necessary! There are always new readers discovering this series.
spoiler
Like this!
Not as action-packed as I've come you expect, but still so immersive. I remember the spider morph on tge original cover was so smoothly transitioned and seemed at odds with the disgusting morphing process described in the books.
This is also pretty smooth, but more organic and representative of how I imagine the morphing process.
