I can asure you they would have both been just as dead if they fought back.
Thats not something you do on a whim, The POS planed to kill them.
How you stop armed robberys like this is simple... you do what the sherrif of pulaski county( the county little rock AR is in) did in the 70s. He had a press confrence saying there would be plain clothed LEOs in som convenience stores, with orders to shoot on site. For the 1st few nights nothing happen, then two people "got lucky" and picked the wrong store to rob. they were both shot and killed. they did not have another armed robbery on a convenience store for 19 months.
You would first have to assure us they would have been acting on a whim, which is something you cannot do.
If the clerks were armed and ready, they would not be acting on a whim any more than the LEOs who were there in the 70s
There was a town in TX back in the 70s too where the Sheriff conducted firearms courses for business owners. At the time it was ridiculed by the other small town just down the road.
after the classes a couple of stores were attempted to be robbed and the robbers were killed.
The robbery rate in that town went to zero while the rate in the town down the road doubled.