The mask could make his life feel threatened, especially with 2 of those in front of him

If he would have gone in bare handed, the 2nd robber could come to the rescue..

The 2nd robber almost came to robber 1's rescue, or was thinking about it, when the knifed robber said that he was dead (So don't bother and save yourself)

Store guy didn't know if the robbers were armed

He chose to be safest as possible without a firearm and went hard on the one he could get his hands on

How high was his being robbed count for the year?

I'll say self defense

Bare handed the store guy could have been the dead guy

Because in situations as this one, these robbers could decide anything

Unless they are dead or in severe condition or scared shitless

The risk comes with the 'job' for robbers