Here is where the concern lies.
Yes, this is pretty much a textbook justifiable homicide and he should not be charged criminally. That's clear. Laying criminal charges requires meeting the probable cause standard, which is a high bar. There are multiple levels of review before this happens (agency and jurisdiction dependent). Conviction, of course, requires proof beyond a reasonable doubt--an even higher standard.
The thing to understand about civil suits is that there is no gateway mechanism. You don't need probable cause and a win only requires a preponderance of the evidence, or 51% wins. The truth is, anyone can be sued for anything at anytime. Something you did, something you didn't do...doesn't matter. It's not uncommon for a suit to begin with a demand letter, which is exactly what it sounds like. Pay us $x or we will proceed with a suit. Often this is enough to generate a payment (hence, why it's done).
While it's true "loser pays" helps, you'd be surprised at how many attorneys are willing to take a shot at a settlement being offered. Depositions, meetings, prep...this all takes time and money. And you're out of pocket for it, even if you're eventually reimbursed.
Keep in mind that the family of the shooter is not the only potential plaintiff. I can think of several others.
Speaking of settlements or jury awards (a jury being twelve people who were too stupid to get out of jury duty), who pays? If you're acting as an individual, you're on the hook for everything. If you're a volunteer security dude at the church, you'd better hope the church and its insurance carrier have agreed in writing to represent and indemnify you. Insurance policies that specifically cover shootings are a good thing, and if you carry you should have one (I do), but they may not cover you if you're volunteering (read: working) to do armed security. My policy only covers me out of state where I don't have police power.
My point with all this is that it is bad mojo to encourage civil action by making reckless statements.