Sulyvanh is a victim of the profaned flame (probably).
After reviewing the lore of the Profaned Flame, I discovered another perspective: that Sulyvanh was also corrupted by the flame.
Description of the Profaned Greatsword:
"Long ago, when Sulyvanh was yet a young sorcerer, he discovered the Profaned Capital and an unfading flame below a distant tundra of Irithyll, and a burning ambition took root within him."
Description of the Immolation Tinder:
"The witches who lead the Pontiff's Knights were originally ordained as holy knights. It was not long, however, before their hearts were swallowed by the Profaned Flame."
Everything in the game tells us that the Profaned Flame corrupts everything it touches; Sulyvanh may have been a completely different person before encountering the Profaned Flame in the Profaned Capital. He could have been a kind and gentle being who, through a cursed twist of fate, ended up encountering a source of evil power that distorted him and transformed him into the wicked tyrant he is today. Looking at it from this angle, Sulyvahn is nothing more and nothing less than another victim of the Profaned Flame, which took root in his soul and instilled in him a lust for power that led him to commit all the atrocities we see throughout the game. Alternatively, it could be that Sulyvahn was always someone with ulterior motives, but who still had moral limits, limits that were removed after finding the Profaned Flame, or perhaps he was always evil and cruel, and finding the Profaned Flame only gave him the power to usurp the throne of Irithyll and expedite his evil plans. What do you think?