This isn't your usual wartime situation. It's not a shooting war or a war of ideologies; it's a war to change our way of life, and as such, it's going to be completely different than the ones we fight.
I view all of the killing the Animorphs did, against the Hork-Bajir and the Taxxons, to be murder, since they're killing helpless innocents, but there's a bit of reason and understanding to it; they're all adults, and I don't care what anyone says about Hork-Bajir, they still have forms of maturity. I don't view the Hork-Bajir as childlike, but simple, so let's get that out of the way. Plus those Controllers were attacking them, so it became a case of self-defense of the "in the heat of the moment" type.
Now, let's take a look at Karen/Aftran. She's a little girl, armed, alone and defenseless, and her host doesn't want to die, and there's a chance her life could be spared, even if only temporarily. Who wouldn't want to take that chance? Really? It's risky to do so, but there are times when you've got to think about other people. Killing children is wrong and it should only be committed unless absolutely necessary. Is it necessary to kill Karen? Like I say, they could take her to the Chee or the Hork-Bajir and she could live there, they might even be able to shelter her whole family if they went with the Hork-Bajir. Plus she's not fighting back, which makes it even more ruthless and all the more evil.
It's not the fact that they wanted to kill her that bothered me; it's the fact that they didn't even consider any other option! Even if they just considered it! Are you honestly trying to say that roughing it in the woods or living underground with mechanical dogs is worse than death?
Like I keep saying, that could have been your relative. How would any of you feel if they killed that relative, a helpless young child, practically a baby, without at least trying to save her life? How do you not try? I'm sorry, but I have these little things called "feelings," and I'd rather die than betray them.