There are several things that are suspect. The extent of the John Brown part bothers me, suggesting a monument in VA to Brown seems a stretch. The part about John Brown being "right" sounds odd because Brown hadn't stayed in some place and simply tried to defend it/promote his views there, but had taken the offensive into Virginia with a Messianic self image. I'm not as inclined to doubt other aspects attributed to his statements: regret for the forays, being able to put himself in the place of those opposing him...although I wonder how forcefully he might have stated them. The idea that it would have been fair play for Brown to take him down while in KS seems reasonable. (On the one extreme he had Edwards acting as mouth piece, and the other end Connelley.)

Another thing that prompted the question was, "I went there to kill Free State men. I did kill them." Did he actually claim to have killed anyone personally while in KS before the war? Shelby doesn't appear to have been one to keep a tally of the men he had personally killed (contrast with Forrest.)

Related to the killing thing is his combative, inebriated testimony in Gallatin, MO for Frank James: "I have lived thirty-four years in this state and never killed anybody yet." This seems a bit unlikely with respect to his very active wartime experience and Mexico, but he might have been using "killed" in the context of murder, or been referring to only his civilian time in Missouri proper. Reading the transcript of Shelby's testimony at Gallatin, it reflects very poorly on him. The Gallatin trial appears to have been a farce, maybe even fixed. If he had to uncharacteristically stretch the truth in other aspects of his testimony as it appears (to me anyway), it would seem likely to have made him very irritable until the matter was resolved.

Shelby's political shift away from the Southern Democrats in later years makes it more challenging to ascribe any fixed view to him. He was known for being a charismatic type with friend or foe and in personal conversation some of that might be from saying elements of what others wanted to hear...and perhaps resulting in them thinking/wanting to believe more had been said than had actually been stated. (This is just speculation on my part.)