If he had succeeded in killing the priest, using whatever method, then the defendant in the forthcoming abuse case (the priest) would not be available.
This is literally a case of shooting first and asking questions later, isn't it.
I think the point with it being a gun crime is analogous to locksmiths who use their skills to trespass and steal.
If you are entrusted with a responsibility that gives you special 'powers' (ability to open doors / kill people at 20 paces) for the percieved common good, then you should be punished more extremely for abusing those powers, than if you had not been a gun owner/locksmith.
I'm with Toad. We try to live in a civilised society. That means having a judicial and political system instead of the rule of the strongest/most well armed.
Even if someone murdered my child in an extreme fashion before my eyes, I could not endorse sentencing leniency if I were to exact my revenge on the perpetrator.
Although as in common with other parents on this thread, I would reservere the right to exact my revenge, but I would accept my harsh sentence with pleasure.