Any previous killings?
A possibility. Could be. But----------------------------
in thinking about SKs, the most common ones coming to mind are men killing females* less frequently males* and most frequently w both sexes, involving sex and physical control. Occasionally women SKs. If he is a SK, he's not typical.
AFAIK, if the D brothers are dead at the hand of Nelson (yes, many of us say pretty certain) this would be his first known killing. He has been previously charged w $-based crimes - receiving stolen property and passing bad checks; he was convicted of some bigger $$$ crimes w loans & w theft of property (selling other ppl's cattle as his own & keeping all the money, instead of remitting to owners, as agreed) and served time in fed pen ~ 1 yr. If he has killed the D brothers, appears circumstances giving rise to it were $$$ related, likely imo selling their cattle and keeping all proceeds.
Others here following Nelson's soc media, such as FB, SnapChat, etc, may have far more insight into him than I do. Or may have other sources of info on him. Maybe he is known in the area for violence against ppl. Anyone searched for that by county arrest records?
My initial thought about Nelson having killed before and being a SK is No, imo, jmo, moo.
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* some young, even children; some older, even senior-age, of either sex.