I'm no expert on this...
I've seen Stewart race a lot, and he has an attitude, but there have been more heated situations for this, if he were the killing type, it would have happened already.
I legitimately believe this was an accident. For all we know he wasn't looking towards Ward and tried to maneuver out of the way last second (accelerating in a dirt car can get it to turn faster), but miscalculated. He might have been pissed, or indifferent, or whatever, but I doubt he was looking to hit the guy.
If he is that sort of psychopath, then he's held it together very well over the years, as he's had *plenty* of opportunity to murder someone in the past.