Another thing to notice is the concept of the acceptable target.
Shoot and kill an alien, and your movie can still pull a PG rating, a PG-13 at the worst if there are worse things as well. Shoot and kill a human, and you have a PG-13 at the very least.
Look at old comic books. Nazis are acceptable targets - in fact, after the World War II, the best times of comic books collapsed due to a shortage of acceptable targets with meaning. Video games - Why do you think we kill zombies and aliens? Because killing humans can be too brutal for the tastes of some.
If you look and any given instance of detailed descriptions of gore and violence in the series, it will almost never be directed at a human - and if it is, it will focus more on the emotional aspect than the physical aspect. The violence happens to the Ani's when they are in morph, or to Hork-Bajir or Taxxons - humans are merely stunned by Ax's tail blade or just knocked out. While the Animorphs attribute it to an unwillingness to kill humans, it's also simply because if you talked about a human's guts spilling out, it's no longer in the intended age group. By the end of the series, humans and Hork-Bajir are on the same moral level for them, but they still refuse to kill humans, even ones that pose immanent threats - dracons, etc.
One of the most important depictions of violence against humans was in MM3, when Jake is shot. The most detail given is "I saw the hole. It was centered in Jake's forehead." And later, when Ax decides to kill Hessians, no explicit details are given. He says what he will do, and the chapter ends with him drawing back his tail, and a "FWAPP" sound. Violence towards humans is almost completely vague or not detailed, even though it can be completely undoubted that a human was killed. Regardless, human death is still relatively taboo when compared to the generous detail given to the deaths of Hork-Bajir or Taxxons, or the Ani's themselves while in morph.
That's pretty much how the series maintained its intended age group - it primarily used so called acceptable targets. Even in real life outside of media we have this concept. Boil a live lobster, and you're a chef, boil a live kitten and you face animal cruelty charges. Our society is very fickle and inconsistent in what it accepts and what is considers wrong - borderline hypocrisy.
PS: I do not condone or in anyway support the boiling of kittens, while I have boiled lobsters myself - After all, I am a part of this society and am susceptible to its practices and ideals.