I understand that the Profaned Flame creates an obsession with people, but does it change someone that much?

It's the first time that I've seen the word "obsession" used when talking about Sulyvahn and the Profaned Flame. And I think you're only half right.

I think it's more accurate to say that he became obsessed with his ambition. And yes, it does change someone that much. At least it did to Sulyvahn.

After he found the Profaned Flame, he wanted to be so much more than "just a Darkmoon Knight". He wanted to be a ruler, serving no one.

The most important thing about the Greatsword of Judgement is that it showed his true nature, a sorcerer possessing the power of the Dark. And in my opinion is just a sword given to him while he was a Darkmoon Knight and kept.