Hmm. I tend to think that it's a bit of a self-fulfilling prophecy: tell a kid he's destined to be a great soldier, and he'll go out and train and fight. Tell her she will be a great leader, and she'll get into politics. Tell them they're outcasts and not worthy of being among polite society, and that will happen.
I think its probably a combination of this, and what Ulphus said in the OP, chances are the mark is accurate enough that there is decent reason to keep doing it. Although IMO, dumping out people who might have mental/socialisation issues and treating them like rubbish is a pretty surefire way of bringing out those issues. Someone who might otherwise have been a productive member of society loses their friends, family, clan, everyone, is it really a huge shock that they turn into a disillusioned thug lurking in the back alley preying on the people that put him there?
The way I play it with Nikita is that yes: there is a gentic/mental aspect of the voluval. She's definitely more violent, and ruthless, then most people, however, her mark definitely doesn't
define her, and its definitely open to interpretation if that behaviour is because of her nature, or because of her socialization, where she was basically raised to be ruthless and violent to survive.
Also, keep in mind that nowhere in the PF does it specifically say
why someone with an outcast mark is exiled. Is it because they're sociopaths? I don't think it is. Why, because the matari employ sociopaths, I mean, that's basically what the Valkears are. Criminals, murders, sociopaths, ones given weapons and governmental permission to inflict misery on their fellow man.
No, I have to think there is something deeper behind this, poor genetic stock perhaps? Or maybe Genetic stock that's too good to let get into the general population? There's all sorts of possibilities.