I on the whole feel like there were many layers unseen to the war and also to voluntary hosts, I don't think they were all conned into being hosts, I think some of them might have been, some of them might not have understood what they were doing, but I believe a few of them genuinely wanted to be hosts, and I also don't think the Yeerks were as evil as they're made out, I do think that many, like Aftran, just wanted to be free, some hosts, like those in the YPM, saw this and wanted to help find the middleground between the slavery of humanity under the Yeerks and the slavery of the Yeerks under the Andalites, and so no, not every voluntary host was a sellout or, as the Animorphs would have it, a traitor.
Chapman was.
He's a sellout for having selfish reasons for betraying humanity on the whole, not once but twice, the first time was due to ignorance and selfishness, the second time was . . . well, pretty much due to ignorance and selfishness.
If his wife threatened his daughter there were other roads he could have taken apart from allowing himself to be infested. He could have run away, taken the girl and tried to disappear, after all Visser Three couldn't even find Loren, it's clear his eyes and arms on earth aren't endless. Chapman might not have gotten far, but then again he might have gotten away, and just trying would have been better than giving up the way he did because in giving up all he did was staple a "don't open 'till x-mas" note to his daughter's forehead, he didn't save her, the end of the war did.
If the yeerks won, which he was helping them to do, she would have been infested, how could he not see that? Besides we don't know what happened to her at the end when the Yeerks pretty much went public, she might have been infested then, what would Chapman have done to stop it?
The flaw with his decision is that apart from willingly helping other people's daughters and sons become controllers just to protect his own for a limited time he has no way whatsoever of stopping them once they win the war. His only defense was being able to make himself look insane in front of parents and that was, for the time being, enough to dissuade Visser Three, once his sanity wouldn't have fallen into question for any reason, or once Iniss 226 destroyed his mind enough to leave him a babbling loon what would his defense have been?
Once the Yeerks won she'd be dead or infested, period. I mean Visser One might have left some humans on earth once the Yeerks had enough because of her kids but Visser Three would just breed more yeerks like he told Elfangor he would, or he'd have torched the planet and killed the last of the humans in an attempt to get the Animorphs, I mean do you really think that's beyond the guy?
It's sort of like a parent who throws their kid out of the way of a train, but lets the kid land in quicksand, they avoid a quick death but if they're not rescued by someone else they just die later after being alone and sad for a good long time, which is pretty much how Melissa seemed, sad and alone until the series stopped remembering she existed.
