It's hard not to compare and contrast this with the Jose Alba case.
On a philosophical level, this is justifiable use of self-defense. That said, it's less cut and dry than the Alba case.
The Alba case was a clear and obvious evil on the part of everybody involved except Alba. Alba was clearly defending himself against a much younger man who was roughing him up and preventing him from escaping. Even in an insane state like New York, it should have been a clear and obvious act of self-defense and Alba should never have been sent to Riker's and certainly shouldn't have been charged with murder.
The intimidation in this case is certainly there. When two men walk into your shop wearing ski masks in Vegas in August, it's safe to assume that something bad is about to happen. That said, the man who was stabbed behind the counter was clearly going for items behind the counter and hadn't made a move toward the clerk yet.
Basically, in New York, I'm pretty sure the clerk would be in jail facing murder charges. Fortunately, Nevada has much saner self-defense laws and it appears that the clerk will not face charges.
This is a case, however, where I'm willing to listen to counterarguments about why the clerk may have not be fully justified while actively trying not to think less of my detractors. If you thought that it was right for Alba to be jailed and charged, you're an idiot or evil. In this case, I just think that you're wrong.