As far as I know, Ritchey never explained himself. I don't have the answer, but here are a couple of thoughts on the subject.
First, the Civil War in Southwest Missouri was a guerrilla war and spawned the sort of brutality that seems endemic to such wars. Reading about it often makes me think of Vietnam. By and large, Union troops kept themselves forted up in the towns and made periodic cavalry sweeps through the countryside to keep the rebels from congregating. They never knew who was friend and foe and acted accordingly.
Christian's violence was not that far out of context in the times, and there was a good deal of name calling on boths sides. For example, one of the men who criticized Christian was Joseph Peevy, who had been the sheriff of Barry County and a saloonkeeper at Keetsville before the war. In 1863, Lt. Col. A. W. Bishop of the First Arkansas Cavalry (Union) described him as the leader of a gang of "marauders of the worst description" and claimed that "no atrocities were too inhuman for them to commit."
Newton County seems to have been suffered more than most areas. The two active Union forces in the county were the 7th PEMM and the 8th Missouri State Militia Cavalry. The local MSM captain was John Kelso, an even more notorious figure than Lt. Christian. Kelso had been a schoolmaster in Ozark MO before the war and was reputed to lie in the brush reading books in Greek and Latin while waiting to ambush passing confederates. His fanaticism on every subject apparently knew no bounds. After the war, he caught his 14-year-old son smoking and the boy was so distraught over the experience that he killed himself.
Second, society seems to have been much more tolerant of violence then than it is now. For 25 years after the war, the newspapers were full of complaints that it was impossible to get a jury to convict a man of murder. If I recall correctly, one of Ritchey's brothers-in-law was responsible for a double killing near Galena KS.
I have read most of the relevant newspapers for Newton County for many years after the war. There were suprisingly few incidents of war-related retaliation.