Bloody Bill Anderson seems to have been your basic psychopath, whose love of killing people was maybe enhanced by being with Quantrill. But probably only in that riding with Quantrill left him more unfettered in his ability to kill others. I used to work with kids who'd been incarcerated and they were pretty tough. But they were all afraid of the one kid who was about 18 and a definite psychopath. They, the tough kids, believed that he was incredibly dangerous and should be held in permanent pre-emptive custody.
The adults who dealt with him also thought he was about the most dangerous individual they'd seen--and they'd dealt with adult criminals in maximum security.
According to what I've read, it's known that Bloody Bill had killed a Kaw Indian before he hooked up with Quantrill. My guess would be that there were other killings by Bill Anderson that were undiscovered.
Quantrill and his gang were common criminals--there's just no way to dress it up. These weren't guerrillas, they were a gang of criminals fighting for nothing more than their own need to be lawless. Some of the gangs may have been men who got caught up in the blood feuds that became part of Missouri-Kansas history and left the gang, but Anderson and Quantrill were just bad guys.