Animorphs, put simply, is a deconstruction of the children's adventure serials that had, as early as the 80s, secured their niche in fiction. In these franchises, which we all know too well from our respective childhoods, a child or a group of children, gifted with extraordinary abilities, defend the universe from the forces of evil. Animorphs, however, attempts to recast those same roles in real life circumstances. The protagonists are children, not heroes, and like any normal thirteen-year-old, they possess neither the know-how, the resources, or (in most cases) even the desire to properly fight a guerilla war against The Empire, which, as one might expect, is vastly superior in numbers and firepower. This is not an underdog story; the heroes are regularly confronted with the inconsequentiality of their efforts, and are furthermore overwhelmed by their choicelessness in the face of the fact that they alone, unable to trust or rely on anyone, are responsible for the continued survival of everything they love, and this knowledge, coupled with their disadvantaged position, pushes them to extremism.
Everything that the series introduces as a cliche of children's serial fiction, it subverts. The Empire is not as irredeemably evil as it appears, nor is The Federation as free, just or compassionate as it claims. The heroes can hardly be described as such, having committed by the end of the story, out of a mix of desperation and ruthlessness, numerous acts that would be considered unforgivable by the average person. You will not be able to find a single character that can be described as completely good or completely evil (except Crayak, and Visser Three).
As Animorphs shows, defending the universe from evil isn't a fun Saturday morning romp. It fucking sucks.