So, if somebody went gun happy on a mormon community before 1976, he could have referred to that executive order and walked away?

Murder is illegal federally. So no.

That’s not likely why they wouldn’t walk away. Murder is illegal federally, but that’ll only come into play if e.g. they crossed state lines in perpetrating the murder.

In all likelihood, the person would be tried in Missouri’s courts – charged with first-degree murder. No judge or jury is going to buy that they actually, in good faith, believed it was their duty to enforce an obscure, long-neglected, controversial executive order from 1838 which directed the military (and not the general public) to exterminate Mormons in the state. If they seriously did (and their lawyer would have to show some very robust evidence), they might get the exceedingly rare insanity plea and go to a psychiatric institution instead. Or, if they were somehow that plain stupid, there’s always ignorance of the law not being an excuse; their intent was still to kill somebody in cold blood. But otherwise, raising this issue as if it’s a loophole is going to piss off the judge and/or make them way less sympathetic to the jury – and possibly even solidify premeditation which first-degree murder requires.