Let me first say, I have no intentions of trying to guess what was going through Stewart's mind at that moment. To state that he had any intentions of running this kid over is a 'bridge to far' for me, without more facts.
But the video looks bad. You can clearly hear Stewart goose the throttle just as he passes by the racer standing on the track. And here is the thing about sprint cars, they have what is know as a limited slip differential in the rear axle. What that means is that when the throttle is quickly opened all the power is transferred to the right rear wheel, thus why the rear end kicked out as it did. This is by design, as sprint car racing is basically a form of 'drifting'. Generally, drivers actually have to steer to the right more than they do the left in order to make a smooth left turn.
But, what a tragic mistake it was on the driver who lost his life to allow his anger to get the better of him, and cause him to walk into oncoming traffic. I'm honestly surprised this hasn't happened until now.