The only way I can think to prepare for this is stay low, try to look inconspicuous/dead, and try to figure out what's going on before taking off running. I'm OK with in open/concealed carry, but playing hero in a situation like this would only cause more problems.

If I heard gunshots in a mall, I would never guess the facts here. This is a jealous husband who went crazy. It's a domestic violence incident. I am willing to bet that the other guy who got killed was the wife's lover, and thus nobody randomly got hurt.