Yes, that word "killed" to me means that the man's death was at least partly the work of another person. I wonder if Garrison was killed for a personal feud or because he was on the wrong side of somebody else's wartime politics.

I looked at the online Missouri State Archives military service records site, but found no service cards for this Henry Garrison with any of the Union militia units in that area, but those records don't include everybody.

I wonder if Henry's wife's North Carolina birth put Henry on the southern side of the argument and local militia killed him as a perceived threat to them.

This is all conjecture without more information.

I recommend you contact local genealogy and history societies in or near Humansville. Also look for those old 1880s and newer county histories of that area to see if they mention your ancestor. The local societies and libraries may have those.

That is about all I can recommend on this.

Bruce