This is a tough one. If the shooter is found to be the aggressor, he is screwed. Unlike police, citizens can’t shoot someone because they slapped them. But after the slap, it will have to be decided why the gun was pulled. If the gun was pulled as a result to being slapped, it won’t be justified. If it was pulled as a result to potentially being pepper sprayed AFTER being slapped, maybe.
If the pepper spray’er is found to be the aggressor, I’m pretty sure being blinded, although temporarily, would be considered serious bodily harm. Similar to if someone was trying to choke you out. The incapacitation may be temporary, but it certainly is reasonable that once you are incapacitated, permanent or long-term serious bodily harm has a high likelihood of following.
Serious bodily harm is usually defined as long term or permanent. Pepper spray causes pain but I've never seen it being viewed as a dangerous weapon.