I don't think he actually gunned the engine until after he hit the kid. IMO, it could easily be construed that he was trying to pass the kid quickly and gunned the engine just as he got alongside him. It's hard to know if Stewart even saw the kid until the last couple of seconds, given the field of vision those cars and helmets allow, and there was another car in front of him. We'll probably never know for sure, but to me, it really does look like it could just be a tragic accident. Kid wanted to go up and scream at Tony as he went by, he got too close, Tony didn't see him until it was too late, gunned it to get by him quickly and the kid got caught under his wheels.
No matter what, the whole thing sure does suck.
FTR, I'm not a Stewart fan, but the pitchforks need to get put away for a little bit. His reputation as a hothead certainly isn't going to help the cause, but it's a big leap from hothead to killer. Going to be a lot to live with for Stewart going forward no matter how it shakes out.