The “fleeing felon” rule was significantly restricted by the Supreme Court in Tennessee v. Garner, 471 U.S. 1 (1985).
Wikipedia has a decent one-sentence summary of the Garner case:
“When a law enforcement officer is pursuing a fleeing suspect, he or she may use deadly force only to prevent escape if the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others.”
From my reading of the press coverage, the predicate for deadly force was completely lacking here.
What a can of worms that opens up. Somebody already driving recklessly could be considered to "pose a significant threat of death or serious physical injury to the officer or others." I don't think this helps at all - could be argued either way before a court.