I don’t think that this attack qualifies under the requirements of the Purple Heart, although I would support the expansion of the requirements to include this. It fails under at least three requirements: death or wounding in ACTION against an ENEMY of the United States, as a result of an act of a hostile, FOREIGN force, or as a result of an INTERNATIONAL terrorist attack.
No evidence has surfaced of a direct link between to killer and a foreign force that would fit the last two requirements. Also, as far as I am aware, neither the Constitution nor statute allows actual citizens to be formally defines as enemies of the state. Even traitors are still only citizens that are unlawfully cooporating an actual enemy.