Looking at this, and considering the fact that concealed carry individuals are required to defend their weapons. I think a strong enough argument could be made that the shooter was afraid of being incapacitated by the gas, to keep a murder 1 charge from sticking easily. If there was audio of the encounter, what they said to each other could also be very important.

Murder 2 all day long though. The shooter initiated the conflict, and had an opportunity to retreat after being slapped, but chose not to. The fact that they were able to aim well enough for a head shot even though the gas was being sprayed in their direction means they were far enough out of range at the time of the shooting to avoid the worst of the spray, and was not visually impaired to the point that disengaging would be more dangerous than standing their ground.