I haven't seen the video or really dug into the evidence to pick a side.
That being said it wasn't "All because one guy felt like he needed to police the parking lot", but rather all because someone who shouldn't have parked there parked there. Lazy, inconsiderate, and mild criminal behavior lead to a series of escalating events ending in the loss of a life.
We can all debate until we are blue in the face about the end results, who is to blame, or if punishment is justified; but to be clear this all began with the actions of the people that parked in the spot they are not allowed to park in. That assumes they are not allowed to park there of course, but I haven't seen any indication they have handicap parking pass.