How intelligent the White Court demons might be is difficult to determine. I suspect there is more intelligence than just that of an animal while in hunting mode. The clue that tells me this is in the soulgaze that Harry shared with Thomas.
A lot about the Hunger Demon and it's hold on it's host was revealed in that soul gaze. The other side of the coin is the picture of a human host, Thomas, that we see fighting the Demon's will. The Thomas of the soul gaze isn't the handsome strong man that Harry knows, but this; Page 169 of Blood Rites
I drew closer. My steps echoed among the pillars. I drew closer to the young man and peered at him. It was Thomas. Not Thomas as I had seen him with my own eyes, but Thomas none the less. This version of him was not deadly beautiful. His face seemed a little more plain. He looked like he might have been a little nearsighted. His expression strained with pain, and his shoulders and back were thick with tension.
Without the Hunger Demon Thomas is a very ordinary man, with very human flaws including perhaps some nearsightedness. On the other side of that mirror is
the Hunger Demon, described as roughly the size of Thomas, humanoid, it's hide shinning with a silver luminous silver glow. Then in the next line is the important bit,
" It crouched , hunched and grotesque, though at the same time there was an eerie beauty about the thing.Notice Thomas and the Demon are fighting through a mirror, not a window.. One reflects the other and what we see in that reflection is the opposite of reality. Which is the real Thomas? The rather plain ordinary man with human frailties, or the beautiful monster? In my opinion this is the source of the addiction, at some point during that first fatal feeding, the frail vanilla human who is predisposed to addiction makes a Faustian bargain with the Hunger Demon, beauty, strength, and long life in exchange for the emotions that feed the Demon. Eventually the human becomes the Demon and accepts it's fate, Lara has for example. Thomas is Margaret's son, he hasn't completely given up the fight, but he is tired.
This is where Margaret says the important bit about Justine. That she is dead, that "It" i.e. the Hunger Demon knows that, and knows her love is the source of Thomas' resistance to it. Justine is dead, and eventually Thomas will become exhausted and the Demon will win. The process gets accelerated later in Turn Coat by the Skin Walker and by the time we hit Peace Talks it is complete. Now tin hat time, the Hunger Demon/ Nemesis created an illusion of Justine, then used Thomas for it's purposes. Once it was accomplished, the Hunger Demon/Nemesis let Thomas know Justine was dead, I believe that is what injured Thomas was mumbling to Harry. The realization that Justine is really dead is what is killing Thomas.. The Hunger Demon/Nemesis has won and has moved on, without the presence of either the love from Justine or the Hunger Demon, Thomas is dying.
Perhaps the demon wasn't using actual words. That could be Harry's projection of what was happening. Even if the demon was only communicating with unspoken ideas and images, that shows a degree of intelligence a bit above being a shark or polar bear on the hunt.
A shark or polar bear are a lot smarter than you think, they carefully select their prey for example.
I don't think this is true, otherwise how did Lara stop herself twice; once while in the Raith Deeps and a second time after Harry blasted both of them out of the Deeps, when she would have been hungry after using so much energy. Lara has learned to master her hunger when she needs to do so or sees a valid reason to do so. The valid reason was keeping her promise to Harry and honoring his guest rights. Of course, if Lara had been starving she probably would not have been able to hold back.
Then again was it Lara's free will? Or simple survival instinct? Also like any intelligent predator the Hunger Demon selects and culls it's victims. I don't think Lara's free will has anything to do with that, as you point out if the Hunger Demon is starving, Lara wouldn't be able to hold back.. I think Thomas has tried or did try that was the struggle that Margaret was talking about, but his free will is weakening.
This means Lara; and by extension, all White Court vampires have free will, unless they are pushed to extremis. Though their demon exerts constant pressure on them, it can be controlled, at least to a degree. This is why Uriel feels sympathy for the members of the White Court. Their free will to make a better choice isn't eliminated but is severely constrained and they are always being pushed to just give in and go the other way.
Or for them, free will is an illusion, they may appear to have free will, but in the end when hungry enough, the Demon will have it's way, the host will feed, feed until death of the victim if need be.. No free will there, it appears that the host doesn't even have the option of suicide to rid themselves of it.. Unless that is what Thomas is really doing now, committing suicide to finally rid himself of it. That's why what Harry saw in that soul gaze was two sides of a mirror, which image of Thomas is real and which one is illusion.