In general, in order to demonstrate that the use of lethal force in self defense was justified, one must show that the assailant had (1) Ability, i. e., the power to deliver force sufficient to cause death or grave bodily harm; (2) Opportunity, i. e., the assailant was capable of immediately deploying such force; and (3) put an innocent in Jeopardy, i. e., the assailant was acting in such a manner that a reasonable and prudent person would conclude that he has the intent to kill or cripple. Under some circumstances, that will be possible even if the assailant is unarmed.
However, to be justified in using force in self defense you must usually also be able to show that you did not provoke the attack, and you may under some circumstances have a duty to avoid the attack or retreat, if you can do so safely.
But reading the newspaper account, this particular situation looks like a giant hairball. Did Hasman do anything to provoke or contribute to the affray? Could he have safely avoided contact with the minivan? There seem to be a bunch of conflicting stories, so it may well be a matter that the DA, a grand jury and/or a trial will need to sort out.