"We don't know "why" the Walmart Shooter stopped shooting and left after shooting/killing "only" 3 people. For an "insane mass shooter" to go to a store and shoot "only 3" random people he apparently did not know and then leave is....very uncharacteristic....of the typical mass shooter."
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Except for the facts that they apparently weren't random and he didn't fit the usual definition of a "mass shooter". They were seemingly targeted for their ethnicity. And such shootings of people by category followed by an immediate retreat is not uncommon among such incidents. Like the couple who walked into a restaurant, shot two cops, and left. Or the "skinhead" who shot a black couple. Or any of a large number of other multiple (but not "mass") "stranger" shootings.