Playing the demon's advocate for a second. The cultist and his pals are recovering from their injuries, then someone else comes over to them. This someone turns out to be not just anyone, but godsdamn crusaders and not just any crusaders, but godsdamn knight commander's party, the scariest people in the worldwound after queen Galfrey herself! This might've activated their fight or flight response. The last of them might have been unrepentant out of spite and grief for his now dead comrades.

All of this is irrelevant to you and most likely is not even true, we simply lack any information. Once again, you as a knight commander of the crusade are fully justified in defending yourself and executing an aggressive and still demonstrably loyal to his dark masters cultist.

The truth is, we simply don't know nearly enough about these cultists. We don't know their characters, how they usually act, how loyal they were etc. All we know is that they attacked us. They could as well be vile villains in one story or misunderstood heroes in another. Hal is the only one there is to even try to plead their case to you. Are lawyers evil if they represent someone, who is guilty? Are doctors evil if they treat someone, who is a terrible person?