Considering the totality of the circumstance, if charges were levied against the store owner, I wouldn't personally convict him of anything, on self-defense grounds alone. No castle-doctrine needed.
Well, I might consider misdemeanor battery for the unnecessary but satisfying pistol-whipping, recommending minimum sentence. But, considering homicide or manslaughter charges, as far as I am concerned, a reasonable person in his circumstance might fear for his life. Outnumbered, and already suffering from being maced, I wouldn't question this fear on the part of an elderly store-owner.
Now, we might all look back and, noticing their actions while the owner was reeling with a face full of capsaicin, say, "no, they might have only wanted the jewelry!".
If bad guys don't want people acting with lethal force on their fears, those bad guys shouldn't go about assaulting folks in such a way as to give those folks such reasonable fears. Far as I'm concerned, the victim's judgement gets the benefit of the doubt, and bad guys just have to live -- or die -- with that.