Damn, damn & damn. Tragic.
Depending on the laws there, if they're similar to some other states, it's not unlikely the man may end up facing an involuntary manslaughter charge.
A manslaughter charge can still be brought even if the man was engaged in the performance of an
otherwise lawful act (justified use of deadly force), depending on the rest of the circumstances.
For example, using a CA Penal Code (just copying what might be the relevant subsection of the code):
CA Penal Code 192PC
Manslaughter is the unlawful killing of a human being without malice. It is of three kinds:
(b) Involuntary—in the commission of an unlawful act, not amounting to a felony; or in the commission of a lawful act which might produce death, in an unlawful manner, or without due caution and circumspection. (underlining by me)
If the law involving the homicide of a human being is similar in
that state, I suspect the police investigation will focus on whether the man handling and shooting the gun, even acting in what he perceived to be the defense of an innocent third person, was acting in a
reasonable manner for the circumstances,
and was using due caution and circumspection in his handling and use of the gun.
When I used to teach classes to private citizens (think CCW licensees), we had many an eye opening discussion of hypothetical situations where what might otherwise be considered a reasonable and lawful use of deadly force might, because of the
totality of the circumstances, potentially result in unexpected criminal charges being brought because due caution and circumspection could be shown as not being present on the part of the "shooter".
Tragic? Absolutely.
As some other posters have opined, sometimes it's possible in private citizen shootings that a lack of relevant training, knowledge of the law and experience could end up putting the private citizen in the position of not being prepared for the sudden, dynamic and possibly chaotic circumstances involved in the use of deadly force.
Yes, of course police also face these same risks, but that's why police training in the laws, and preparation in hopefully using good judgement in making critical decisions, including not only being able to rapidly decide that a use of deadly force is not only lawful, but reasonable and appropriate for the circumstances.
Police also have some additional protection in the form of qualified immunity while invoking their authority for acts done in their official capacity, presuming their actions are not only within the law (which includes relevant examples provided by existing case law), but also within agency policy. Acting within policy can be just as important as statutory law when it comes to the actions of police (yes, this is a gross simplification of things).
This is where police, if acting outside policy, can expose themselves to greater liability (and sometimes provide an opportunity for agencies to distance themselves, to some degree, from the actions of an officer deliberately acting contrary to policy).