These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternate Character Interpretation: The whole book does this to numerous characters as opposed to their traditional portrayal. Also, Derfel very grudgingly admits at one point that his personal view of Lancelot could be clouding how Lancelot comes off in the story
Gundleus is one of the first villains of the series. Initially a lord sworn to King Uther, Gundleus is also wicked and ambitious and has no intent of honoring his vows to serve his dead king's infant son Mordred. After the death of Uther, Gundleus approaches the baby with its young mother and then murders her after demanding she kiss his sword for protection, before stabbing the infant as well (unknown to him, the infant Mordred was swapped out with a decoy). He then proceeds to rape Merlin's disciple Nimue in retribution for her attitude towards him and rips out one of her eyes for spite. Even his affection for his lowborn lover is shown to be a farce as he abandons her when he gets a better betrothal offer.
King Dwirnach of Ireland is a vicious tyrant aiming to conquer and kill. What sets Dwirnach apart from the other warlords of the series is his love of Cold-Blooded Torture as a hobby. Dwirnach prefers to take his enemies alive, so he can spend his time slowly flaying them. He even keeps the skins as perverse trophies and threatens to both rape and flay the hero Derfel's lover Princess Ceinwyn solely to satisfy his lust for pain.
King Mark of Cornwall is well known for marrying much younger women and killing them later when he's tired of them. When one of his betrothed, the young Isolde, escapes Mark with his nephew Tristan, Mark hunts them to Arthur's lands and demands satisfaction. Unable to break the peace Arthur is forced to compromise his morals by allowing Mark's "justice," allowing Tristan to engage in a fight he cannot win against Mark's champion who reluctantly kills the valiant knight. With Isolde back in his clutches, Mark immediately has her burned alive on the beach by where her beloved died before returning to Cornwall with smug satisfaction to continue his practices.
One-Scene Wonder: Merlin has relatively little time directly interacting with Derfel in the books, but boy does he know how to make it memorable.