Maybe we should define Free Will.
Let's talk about humans. A teenager who is the sixth generation of a military family, who has been to military school his whole life, could wake up one morning and decide to dropout. Head down to Burning Man and chill. Another teenager, one with hippy parents, could decide to ditch the peace and goodwill bullshit and join the army.
Neither choice is likely but both are possible. While people are the sum of their experiences they still have that spark. They can still wake up one morning and turn their lives around. To not just think outside the box but to jump from their current box to another one. To make an effort to become a radically different person.
Mab is Mab. She can do a lot of things but she can't dropout - she's tied too tightly in a web of obligations. What she does defines her almost as much as who she is. She's incapable of making certain decisions.
The average vampire is the average vampire. Joe the Black Court vampire can decide if he's going to stalk victim A or victim B, if he going to pursue this plot or that plot, but he has to stalk and he has to pursue plots. He can't just take off to Burning Man. His existance as a Black Court Vampire defines his choices as it defines him. An investiment banker could decide to give all his money to charity and join a Buddhist monistary but Joe is always going to be a Black Court Vampire. An aging hippy could take up a gun and start shooting everyone who doesn't believe in peace, love, and brotherhood but a Black Court Vampire can't to turn his (un)life around.
It's like having the choice of doors A to Z but you can't even think about using a window - or leaving those doors behind and dancing in the grass. All of those choices can give the illustion of freewill but only the illusion of freewill.
That said, the illusion is usually enough. Seriously, Harry doesn't say "Look what Mab's nature is making her do" but "What is Mab planning now?". She might not have freewill but she has enough choices available to her to fake it. Joe the Black Court Vampire doesn't have a world of options available to him, but since he can choose whether or not to stalk A or B, to fight or retreat, he has enough choices to fake freewill.
One of the best "freewill verse monster" descriptions in the DV is in the short story
Even Hand (which is narrated by Marcone). To quote from that story:
I am a professional monster.
It sounds pretentious. After all, I’m not a flesh-devouring ghoul, hiding behind a human mask until it is time to gorge. I’m no vampire, to drain the blood or soul from my victim, no ogre, no demon, no cursed beast from the spirit world dwelling amid the unsuspecting sheep of humanity. I’m not even possessed of the mystic abilities of a mortal wizard.
But they will never be what I am. One and all, those beings were born to be what they are.
I made a choice.
And that's the issue of freewill in a nutshell. If Marcone wants to change he can. He could give all his money to charity and devote his remaining days working at homeless shelter. He could read to the blind or otherwise change his life for the better - he just has chosen not to. He is what he has chosen to be - but he still has the choice to change. Of course, he has enemies. If he did turn his back on who he is decide to become someone else then odds are he would be dead in days, but knowing that he could decide to reform.
Mab doesn't get a chance to reform. Joe the vampire doesn't get a chance to reform. Only Marcone gets to do that because he's the only one with freewill.
Richard