This question reminds me of the time I was STing a World of Darkness game and the discussion among the group turned to one of the PCs being a serial killer. I was against the idea precisely because of this issue of Free Will and acting against your nature. I didn't put it in the words of the Dresden Files game, but when I read the game, I immediately understood what they were talking about.
After I was able to establish that Dexter is absolutely utter fiction, I was able to get them to listen to my points. I wasn't claiming any special knowledge as a psychiatrist or anything, but I am sort of a serial killer-phile. I explained to them that serial killers kill people. It's what they fantasize about, it's what they think about, all their goals revolve around get hold of a victim so they can kill them in whichever manner they prefer. While you plan to take vacation from work or school, they plan how to kill someone. While you have aspirations of growing up and being an astronaut, they have aspirations of killing someone. While you adopt a pet for affection and companionship, they adopt a pet to practice killing. Sharks swim, bees buzz, serial killers kill. It's a compulsion.
They argued with me for a long time, too. In retrospect, I'm grateful for their attempt to empathize with the likes of a serial killer. I blame Dexter for that sort of thing. They would counter me with things like "Serial killers has life goals and aspirations outside of killing people. Look at Ted Bundy -- he was in college! That's proof that he wanted something more out of life." And I was like, "Yeah. Who were Ted Bundy's victims?"
It's like saying, "Look at Gacy! He was a part-time children's entertainer. That's proof that he aspired to bring joy to the world!" And I would be saying, "Yeah. Who were Gacy's victims?"
If that's the kind of character you want to play, then go ahead and play. I'm more libertarian when it comes to letting people play what they want to play then I was back when I had that debate. Also, the FATE system is built to accommodate playing a character with no soul, a character with no Free Will, a creature of their nature, a creature of compulsion.
Say that I was your GM and I wanted to prove a point to you about playing a "monster" with no Free Will. Every time you tried to do something to help the group or to further your goals, I Compelled your Aspect BLACK COURT VAMPIRE so that you killed your contact, your ally, your friend. If not kill them, at least attack them and attempt to eat them. Just when you thought it was safe to go back into the water...
What would you do?