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<p>Vigilantism does not necessarily require planning or rational thought. For example, rival criminal gangs might attempt vigilante attacks against each other based on encountering an opportunity, rather than planning them out beforehand.</p>
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<p>Was the female suspect already wounded and down when he fired the killing shot? If so, then the self-preservation motivation becomes weaker, because the escaped male suspect whose whereabouts were unknown and could theoretically return with more firepower would be a much greater threat. Indeed, it may have been better to not use any more ammunition on the wounded and down female suspect in order to have it in case the male suspect returned and needed to be resisted.</p>
<p>In any case, the law in even the more lenient states for self defense stops short of clearly making the killing in this scenario legal. While a conviction may be unlikely (and the DA may not prosecute at all), it is not so clear cut as you describe.</p>