I'd rather watch The View than Nascar, so I don't have any horse in this race.
It's completely, unequivocally silly to even mention criminal charges in this matter.
Here are the facts:
Dude wearing black outfit and black helmet gets onto dark track and puts himself into the potential path of oncoming vehicles, which he knows are there.
He intentionally puts himself into close proximity with a car driver.
He is hit and killed by that driver.
The video doesn't show Stewart "speeding up" or "driving at him." If anything, he was worried about hitting this Darwinian Exhibit A and steered left, causing his ass end to kick out right.
No reasonable person could conclude, beyond a reasonable doubt, or even close to that standard, that Stewart intentionally or recklessly hit this idiot.
Not to be put too blunt a tip on it - but you either have an axe to grind against this guy or you're not a reasonable person if you look at this and think that this is criminal liability here.
And this is what is scary about the justice system - get a DA looking for exposure or a LEO with no common sense and maybe they start the process and this becomes a huge waste of everybody's time.