You're not paying attention to the timeline here. At the end of Turn Coat, Thomas was just days removed from the Nagloshii's torture. That was him at his lowest point.
The passage I posted is from Ghost Story, so Thomas has had several years to heal and regain his balance. He was much closer to being his reformed self in Changes and in GS he was dealing with Harry's death along with the constant Hunger.
But he wasn't going out and being a predator, he was denying his Hunger and suffering for it. He's still trying to do the right thing but is subject to a relapse just like any other recovering addict. He's making the right choices but the wrong one's whisper strongly to him.
That's true. It's not that Thomas doesn't feel guilt at the end of Turn Coat, he does. If he didn't, he'd be at peace with it, relaxed. Instead he's obviously bitter and trying to escape his own conscience.
As Uriel notes, Thomas has a choice. But Thomas is not alone in his head. The demon is in there too, and it both is and is not a separate thing. As long as Thomas is awake, he has free will and can potentially choose not to act in accordance with his vampiric impulses. But when Thomas is suppressed, when the demon is the only guiding mind (I'm actually not sure how intelligent the demon is, it seems almost like an animal in some ways), then Thomas' free will isn't relevant because Thomas is not Thomas at that point. Thomas is asleep, or unconscious, or whatever we want to call it.
It's like the
loup-garou. MacFinn isn't guilty for the murders the monster commits, but the danger is still very real. MacFinn has free will, but the
loup-garou suppresses MacFinn, so his free will isn't in play when the monster is loose.
What happened to those girls that Shagnasty fed to Thomas is not Thomas' responsibility...mostly. There is an element to it like the loup-garou, where the human side has an obligation to plan for situations to prevent the monster from being able to murder.
But there are two perspectives to it. There's the abstract moral responsibility, the distinction between the human in the WV and the demon. And then there's the practical perspective of the potential victims.
That's what Eb is getting at, what Bob was trying to warn Harry about back in the day. From the POV of view of a human, any WV is a bomb waiting to go off, in much the same way as the loup-garou lurked within MacFinn. In fact, it's worse in way. The loup-garou only manifested when a certain trigger condition was right, and it was
predictable. The full moon comes on a regular schedule, and preparations can be made. The White Vampire demon can get loose at
any time the WV is Hungry enough, or otherwise weakened.
(I suspect that one nasty move a warlock could try using mind magic would be to weaken the human mind enough for the demon to come to the fore at a bad moment.)
From the mortal perspective, it hardly matters if the monster attacking and devouring them is Thomas or Thomas' demon. Their life and their sanity are in danger either way,
and where Thomas goes, so goes the demon. It's
always there, always present, always waiting to get loose.
Which in turn makes associating with White Vampires, being friends with them, dating one or marrying one or just hanging out with one, risky. Eb and Bob have both tried to warn Harry about this over the years. He's in denial about it somewhat. Remember his reaction to watching Lara drain Madeline while tearing her apart physically. It's really easy to forget that White Vampires are monsters, when you're used to their civilized side.
Thomas regained enough of his balance, after some time passed, to regain his normal relationship with Harry and to try and behave decently again, and he probably works harder than ever to keep his demon on a leash. But notice that he quit trying to live a normal human life, too. He feeds more regularly now, probably out of fear of getting too Hungry and it getting loose again. He lives with the White Court again, he's no longer trying to pretend to himself that he's not what he is, at least to that extent.
Is that good? Or bad? Or both? I'm honestly not sure. You could make an argument either way on a reasonable basis.
I mentioned above that it's mostly not Thomas' responsibility that those girls are dead. The intent lay with Shagnasty, Thomas was the target, the girls were the collateral damage. But if Thomas wants to be a 'good person', he does have a moral responsibility to recognize the danger he himself represents to other people, just as a loup-garou host (at least one who knows what he is) does. He has a duty to take what steps he can to make sure the demon never gets into control again. If he doesn't at least make a solid effort in that direction, then he does share some responsibility for what the demon does if it gets loose.
If I discover that I am under the loup-garou curse, and I don't take any precautions and I'm careless about the moon phase and so forth, then yeah, I'm at fault if the monster kills someone. Not as much at fault as I would be if I shot them with a pistol, but in that scenario I would not be innocent. A WV who wants to be 'good' has the same challenge.