In looking at the 1889 County History of Hickory, Polk, Cedar, Dade, and Barton counties, Missouri. On page 319 you may find your answer. It states
"During the war, a number of men were killed in Polk County by guerrillas and bushwackers. Some of them were Northern and some of them Southern sypathizers...The story of the death of Dr. Weaver is a sanguinary one. He had been South with his nefores and stock, and had returned with considerable money. On the night of August 13, 1863, a body of armed men appeared, and shot him as he lay in bed. Seven balls entered his body, the first near the heart, being sufficient to cause instant death. Robbery is believed to have been the motive for this cowardly assassination." This might explain the death of Henry Garrison as well about that time if these armed men paid him a like visit as well. Might be worth a search to see if you can find Dr. Weaver's grave, and see if he is listed ad Killed instead of died.

Hope this helps,

Michael