First let me say that the MC very shortly won't give a flip about what the other Vampires think. Virgil already senses his power and he is behind the Princeps. Those of any importance in the Vampire Hierarchy see him for what he is.Since you choose to focus on that point If I remember correctly Laurrie and later Ivy were already known to the other Vampires.
And I believe the reason given was leaving them unthralled risked Vampire Exposure.
Carman was part of the Vampire Hunters and they already know about Vampires.
Have done for hundreds if not thousands of years and aparently are not telling the rest of the world.
Carman is not as far as anyone knows yet known to anyone out side the MC's circle.
And even If she was thralled once she was known about it would not help either the MC or Sharon.
The other Vampires wouldn't care or except her to still breathing even if she was thralled.
So MC and Sharon are screwed either way.
Her being thralled would not benefit her or them against the other Vampires.
Just her being Vampire Hunter alone would mean either instant death or at the very least she would end up like Gregor.
Thralled or not aint gonna save her or them so again whats the point in thralling her at all?.
P.S.there was two exception to that circle Astrid who may know but for what ever reason chooses not to tell.
And Calisto who may of fed on Carman but doesn't seems aware she fed on a Vampire Hunter.
Just Sayin.
I personally would think that Carmen would in fact be a feather in his cap so to speak. He now has a mole into the Vampire Hunters organization. This has already paid off with the information about the blade.
To the point of Astrid and Callisto , Astrid is the reason that the MC got Carmen in the first place. If anyone is to be killed it's Astrid as she turned on her own. And as far as Callisto is concerned anything that is of any entertainment value she is al for it.