One of the most interesting ways to study Quantrill is to read the memoirs of his men. When T. J. Stiles quotes Quantrill at Lawrence as saying: "Kill and you will make no mistake", he is really quoting William Elsey Connelley, who in turn is quoting William Gregg. Gregg wrote his very interesting manuscript at the request of Connelley. He very successfully fought a rear guard delaying action as the guerrillas retreated from Lawrence back to Missouri. It seems clear to me that he was disgusted by some of the abuses he witnessed at Lawrence. In any event, he then left Quantrill and went into regular Confederate service with Shelby. You can read a transcript of his manuscript here:
http://penningtons.tripod.com/charleythehorse2665-2.html
John McCorckle was a scout and guerrilla fighter in George Todd's band. He went to Kentucky with Quantrill and eventually surrendered there. His memoir is titled "Three Years With Quantrill". It is flowery at times but not nearly so much as John Newman Edwards's writing. I suspect his recollections were sometimes clouded by his advanced age at the time he dictated the work. I have the edition that was annotated by Albert Castle and he is frequently critical of the dates that McCorckle assigns to events. Nevertheless, McCorckle gives very interesting character studies of several of the boys, including Quantrill. You can download the book in the format of your choice for free here:
https://archive.org/details/threeyearswithqu01mcco
Hamp Watts wrote a brief, excellent memoir of his time in Anderson's band, from 1864 to war's end. It mostly covers Anderson's actions in Howard and Boone Counties in 1864 and 65. It references Quantrill frequently. It's called "Babe of the Company". You can read it in its entirety here:
http://statehistoricalsocietyofmissouri.org/cdm/compoundobject/collection/amcw/id/15641
As has been said, Connelley is fascinating, too. He befriended numerous surviving guerrillas in the years after the war. He wrote about them very fair-mindedly. He most likely fabricated most of what he wrote about Quantrill's childhood, but he is able to relate the other boys' eyewitness accounts of Quantrill's wartime actions. I have not seen the Kindle version of the book, but the original is now available in scanned, PDF form. I have said many times that the voluminous footnotes are every bit as interesting as the main text. You can download the book in your choice of formats here:
https://archive.org/details/quantrillborderw00connuoft
There are also accounts drawn from the oral tradition of Bud and Donnie Pence that provide interesting character studies of Quantrill and many of the boys.
Reading these and other accounts, I've gradually come to the conclusion that Quantrill had a strange duality in his character. He was smart, cunning, charismatic and very chivalrous at times. He was cold other times. I believe he lost control of his command because he was not ruthless enough to suit Anderson and Todd. He is usually demonized for the massacre at Lawrence, but I believe Anderson and Todd and some of their minions were more to blame for that raid getting out of control (young boys being killed, etc.)
I think Quantrill was a very good leader. He managed to operate successfully for years in hostile territory. He disrupted troop movements, captured or destroyed enemy supplies, sabotaged bridges and railroads and typically confounded the efforts of militias and regular troops to capture him. He took and paroled captives. He protected a group of civilians at Lawrence. He shot a fellow guerrilla for attempting to steal a horse from a Howard County widow. He was a complex man. He attempted to get a commission from the Confederate government (accounts vary as to his success). He might or might not technically have been a soldier, but I'm sure he considered himself one. He was a classic partisan guerrilla. The more I find to read about him, the more interesting he becomes to me.