This tells a story.

From what I can gather the victim slapped the shooter, at which point the shooter aimed his gun and cocked it. The victim and shooter used the mace/fired the gun at the same time.

This was all happening while the guy in the Black Guns Matter shirt was agitating the crowd. The shooter likely tried to intervene somehow and got slapped as a result, at which point the rest happened.

Getting slapped is not justification for shooting someone in the head.

Judging from the position of the shooter’s left hand in the first picture he appears to have been trying to grab the victim’s mace. Furthermore, he fixed his glasses after the slap and before shooting.

Following the slap, the victim appears to have also retreated before the shooter fired.