Note that this is only speculation, but it should be good enough. In essence Rogue Knights are the free souls of Pendor, free in the bad ways of the word. Rogue Knights hold human lives and morals in very low esteem. They only live for money, booze, women or whatever they happen to be after. Anything they want they take whenever and from whoever they like. They hold little or no respect for authorities when in pursuit of their goals. To sum it up, they are the pillagers that only think of themselves. This is what distances them from Adventurers who usually have some standards that guide them. Rogue Knights tend to come from richer families (you need to pay for that equipment and get the training to use it somewhere you know), unlike the less lucky bandits.
Remember: speculation.