So, it appears that Avington's dad Avington Simpson was the one killed and this took place in June 1863. This is exactly what I suspected.

In my "Guerrilla Warfare in Civil War Missouri, vol. II, 1863" on pages 162-3 I described an unidentified Union murder team that worked off a "kill list" of known southern sympathizers then living in several counties around the large Union base at Springfield during the middle of 1863. Since Polk County is immediately north of Greene County that contains Springfield, I suspected as much even before Dr. Russell sent his reply giving June 1863 for your ancestor's murder. These soldiers from the Springfield garrison in uniform rode out to each victim's home and assassinated each man usually in broad daylight and in some cases in front of his family. The endnotes #21, 22, and 23 for Chapter 12 on page 342 list some of the sources I used, but basically I obtained this information from folks like you who placed inquiries in genealogical forums. If you look at my bibliography on page 363 at the back of the book you will see records of our email exchanges for men killed:
-- on 3 May in northeast Polk County......... (two Pitt men & a Brannon);
--on 16 May in Lawrence County............... (Brashear);
--on 11 July in southeast McDonald County.... (Schell);
--on 15 July in south-central McDonald County (Laughlin); and
--on 1 September in south Polk County........ (Burns).

Adding your ancestor's June murder to the list, you can see that these unidentified Yanks were active from late spring through early fall of 1863. It makes you wonder how many other men they assassinated who we haven't heard from yet. This is one of the reasons that large portions of southwest Missouri were uninhabited by war's end. We all hear about "conspiracy theories" and how people exaggerate such ideas, but it appears this one seems to be genuine.

Bruce Nichols