Tony Stewart -- Hits and Kills Another Driver During Dirt Track Race

7:45 AM PT -- Stewart has pulled out of today's Sprint Cup race. His team says Regan Smith will fill-in behind the wheel of Stewart's #14 car.

NASCAR superstar Tony Stewart drove his race car into another driver Saturday night … sending the man to the hospital where he was later pronounced dead.

Kevin Ward Jr. had gotten out of his #13 car after an on-track accident involving Stewart during a sprint car race (dirt track) in Canandaigua, NY.

20-year-old Ward appeared angry after the wreck and attempted to point and yell at Stewart as he drove past in his #14 car on the next lap -- but Tony ended up hitting him … sending Ward flying down the track.

Hours after the accident, cops said they were investigating Ward's death as an "on-track" incident, and not a criminal matter. But it could turn into one if the evidence warrants.

Stewart was supposed to race Sunday in the Sprint Cup race in Watkins Glen, NY.