It is not about what is nice but what is necessary.
Scenario 1: Yeerks invade in an open war scenario, far better is the species that occasionally torturers traitors/voluntaries than the species that invaded their planet and killed billions of people.I think to preserve earth's natural environment, human art and literature, freedom, democracy, joy and love basically everything that one loses if you become a controller would justify anything. It is not the tactic but the cause that really matters (within reason and with common sense and within proportion, i am not justified in torturing/nuking New Zealand over a fishing dispute, but nuking hiroshima was acceptable given the circumstances of world war 2 and probably saved Japanese lives as well.
There are degrees and distinctions, a holocaust denier (I think it was Diana Mitford) eventually said that it didn't matter if Hitler killed 6 million or 6 people because 1 is still taking a human life (roughly) who ever said this was an idiot, degree does matter, how far and why you do things matters.
Scenario 2: Yeerks cut a deal and the human race is forced to compromise.
People like Kharina and Terenia (and all the other people who said they would become voluntaries) are probably really nice, intelligent people in real life (i am guessing on the basis of numerous posts) and in such a situation they might volunteer themselves out of pity for the yeerks or ideas of self sacrifice. This would be a tragedy, as they have a lot to contribute to the human race as individuals and as parents/members of the human gene pool. Since Yeerks who are able to infest Gedds and Hork-Bajir just fine (barring a few linguistic problems) don't require their hosts to be law abiding, intelligent or sane as a yeerk basically overrides a hosts brain and controls actions, the intellect of the host is irrelevant it may even damage them (as people discuss do high intelligence hosts drive yeerks insane) therefore it would be a tragedy if someone as intelligent and as good as Kharina where to become a controller, none of her great characteristics would matter or be useful to anyone, far better that if one had to make this terrible compromise with the yeerks, that you give them the people who cannot contribute and whose presence in the gene pool is damaging... There would be potential for abuse: originally just criminals, then retarded people, then the profoundly mentally ill (rather than just people who are little depressed) then the welfare dependents, then the third world etc... but barring that sort of sliding slope, it would be a win/win situation: Yeerks get dumb, easy to manage hosts, humans pay fewer taxes and don't have to worry about Earth turning into Idiocracy...
Who is Yorick?
Well, actually the two species are about the same. Consider- the human that hurts a voluntary has hurt another member of their OWN SPECIES. At the same time this human has probably eaten battery farmed meat or used a medicine tested on animals or done something similar. The Yeerks, when they kill or infest humans, are only hurting a member of a different species- an animal. As Aftran said: "You're our meat!" As humans do this all the time, and many of them only do it for fun (all those sadists who'll set a kitten on fire etc.), we can't exactly take the higher moral ground here. Yeerks do however often hurt other Yeerks- just take v1s starvation which was a public execution and was somewhat similar to hanging drawing and quartering- similar pain levels.
So humans and Yeerks are remarkably and astoundingly similar. In fact, on their respective planets they're both relatively physically inferior (OK we're not quite as bad as Yeerks but compared to many other animals on Earth...) but have achieved a dominant status through intelligence and through using other species and machinery to achieve what they physically cannot (dogs, horses, oxen etc. in our case, gradually replaced by tractors, cars etc.)
OK, not sure how that started with me ranting at voodooqueen for senseless cruelty and ended up with me comparing humans and Yeerks. Sorry about the rambling.
Well the yeerks certainly do get a lot of unwarranted woobiefication. I can understand them and i feel sorry for them, being blind slugs and all, and it's not like the average yeerk civilian was deliberately melicious. But in the end invading another planet is always wrong and the defenders have blank cheque when it comes to permitted retaliation.
What the Yeerks did was wrong, but it does not give us 'blank cheque' for retaliation. We shouldn't retaliate at all, we should do only what is necessary to defend our planet. Just because someone is in the wrong by insulting me or even hitting me does not mean I am entitled to murder them and torture and murder their kids. I would be entitled to grab them and hold them back to stop them punching me again, or someone else would be entitled to do that for me. If they couldn't hold them, they might be entitled to knock them out. But no more than they needed to to stop them being a threat.
It's the same with wars. If someone threatens you (or someone else), you defend, but you don't start committing acts of revenge.
Or if you prefer, the old 'two wrongs don't make a right' saying that my parents were always fond of quoting to me
I'm not attempting to absolve the Yeerks or the voluntary hosts of all blame. It was wrong for .e.g. Taylor to do what she did. But the whole point of Animorphs is that KAA pretty much always strives to make us understand WHY. With Aftran, with Taylor, even with Visser One and Visser Three. We maybe still don't agree with what they did, but we can see where they're coming from and empathise with them.
Finally, what you mentioned above about having aliens as enemies meaning you can kill and kill and never give them any humanity- might be the case in Star Wars or even something like Lord of the Rings where no-one ever considers that orcs might have feelings- but reversing that idea was one of the main points of Animorphs. It was the WHOLE POINT of books like number 19. I think it's pretty obvious that KAA spent the whole series of books trying to make us see shades of grey.
The first time I read number 19 I was yelling at the book: "Just kill her, she's a Yeerk for heaven's sake! If Marco can nearly kill his own mother to get the Yeerk, you can kill some random kid you don't know!"
I came out of the book with an absolute Aftran obsession. Now that's good writing
(You can tell I have an Aftran obsession by the fact that this long post came back to her in the end. this often happens).