The Hork-Bajir were a decent pacifist species, with their own sentience, yes, with their very own families, yes, but physically strong and lethal, and the Yeerks took advantage of this. With a 'slight' exception of Rachel, the Animorphs DID avoid them where possible! This was also obvious!
A rule against killing a human-Controller was about not losing sight of what they were fighting for. But I think a rule where they shouldn't kill Hork-Bajir-Controllers would be self-sabotaging.
I think the quote from #30 adds to what pretty much everyone has already said:
....
"And yet..." Visser One mused. "And yet, the casualty reports from Earth are always weighted heavily toward Hork-Bajir and Taxxons. In fact. . . I am trying to recall when I have ever seen a report listing a human-Controller casualty."
My guts were ice.
We had made a mistake. We had made a terrible mistake.
<What do I say?> Cassie demanded.
<l . . . I . . .> My brain wouldn't work. The thoughts wouldn't form into any sort of order.
Visser One had just put her finger on our greatest secret.
<Say something!> Rachel yelled.
<No, too late,> Jake interrupted. <Too late. Let it go. No choice.>
"Well, well, well," Visser One said.
She knew.
There was only one reason why a group of Andalite guerrilla fighters would inflict more casualties on Hork-Bajir than on humans: The Andalite guerillas weren't Andalites.
A human would spare a human life.
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Well, they were technically playing along as 'Andalite Bandits' fighting the Yeerk campaign on Earth. So, in Visser One's P.O.V., there should have been just as many casualties on the human side. Being a resistance for Earth, they should have responded something like <Perhaps we are not as willing to kill the people we are trying to save as you Yeerk filth are.>
But then the Visser would have spat some hypocrisy over the Hork-Bajir Homeworld rarara...
Damn! That moment really bites me. It's really hard to pick what they could have said there.