Cases in the past have required a very low threshold for "threat".
For example in the case State v. Simon, a man (Simon) who shared a duplex with an Asian man (Wong) shot and killed Wong as Wong attempted to enter his own home. Simon claimed self defense because he was afraid of Asians moving into the neighborhood and was afraid that Wong knew martial arts. Simon claimed Wong walked towards him cursing before the shooting but evidence showed Wong was at his own door to the duplex. The jury found in favor of Simon and said the killing was justified.
Similarly in the case State v. Peairs, a man (Peairs) shot a 16 year old exchange student (Hattori) who had rang his doorbell mistaking the address for a party he was attending that night. Peairs wife first opened the door, saw that Hattori was a stranger (allegedly dancing in I believe a Risky Business costume on the doorstep) and yelled to her husband to bring his gun. Peairs shot and killed Hattori and was acquitted because his wife was found to have reasonably believed Hattori was a threat. Peairs was actually so devastated after the facts came out that he swore to never own a gun again.
In neither of these cases did the person claim they had actually been threatened.
If M himself reasonably believed his wife was threatened, he was allowed to exhibit force, JMO of the law based on the hundreds of cases, papers, studies, etc. I have read. I actually have a published journal article on the topic of self defense law.
Now thankfully the laws have changed somewhat in that the "reasonable belief" is supposed to be objective in the circumstances. It is still applied subjectively, though, IMO, by numerous jurors and police departments. Which is what I believe happened here with the sheriff. And I truly think if this case goes to a jury it could come out either way. I just personally do not feel the killing was justified. I do not think the situation should have escalated to deadly force.