... you have to come up with how the clerk could have made an assessment in two seconds as to the intent and criminal history of the fellow coming towards him.
...
Would you just hope for the best and think the person coming at you intended you no harm?
Why & how would the clerk be in any way expected to assess (let alone know) the criminal history of the suspects? Does
previous criminal history of one or more suspects relieve the clerk of the requirement of acting lawfully in the present?
Regardless of what you decide to do, if your actions aren't subsequently determined to be necessary and reasonable under the law, you risk your actions not being considered justified under the law, and risk being held liable for unlawful conduct. (Perhaps followed by bearing responsibility under whatever civil statutes may be in place, too.)
Nobody ever said these things were simple and clear cut (far from it), or that everyone would always make the same right choices in bad situations.
Even with
many, many hours of training, both at the academy level, as well as in-service (FTO, recurrent training, legal updates, etc), even LE can make the wrong calls in a crunch. Further, due to their higher level of training and expectation of a higher knowledge and the ability to apply that training and experience, they're often held to a higher standard than
other private citizens, and may receive a heavier sentence, if found guilty. Then, the civil (state) and civil rights(federal) repercussions can be mind-boggling.
Even without knowing the laws of the state where this tragic incident occurred, I'd not be surprised if the store clerk accepted a plea deal for a reduced charge. If the store is insured, the company is probably going to pay out in a substantial manner, even if the defendant is tried and found guilty (that can be the nature of civil actions). Depends on the way the laws work in that state. Dunno.