While not canon per se, the books all seemed to paint the Black Knights as a pretty violent and evil group committing crimes in recent times. This is pretty indicative of them as a group I'd say.
The books are not canon in the slightest when it comes to the game lore, so any crimes or violence they commit there cannot be taken as indicative of what the organisation is within the game.
The books are canon. Several in-game sources verify it. The Black Knights were the aggressors in the last war, and their previous leader, Sulla, was a ruthless, violent, cartoonishly villainous psychopath.
The Black Knights are also slavers. They totally use slave labor. That's not something T.S. Church came up with, either, it's straight from in-game lore. You gonna defend that too?
How many evil deeds have the White Knights committed in canon? I'll give you a hint, it's none, because they're the good guys.