Well, in Houston at least, you can volunteer: http://www.houstontx.gov/parking/volunteer.htmlNote to self: not your job to police handicapped parking spaces.
As for whether or not the shooting was justified, I think they both screwed up, but watching the video several times, Markeis (the shootee) advanced on Drejka (shooter) after pushing him to the ground. He only backed off when he saw the gun. I think it's reasonable to assume the aggression wouldn't have ended with just an [uncalled for and very hard] shove if Drejka didn't have a gun. I think Drejka was justified in pulling the gun. Firing it, that's a bit harder to judge, but I'm leaning towards no.