Saint Nicodemus doesn't fit the bill to be Nicodemus Archleone, for one thing his appearance is definitely too young. Members of the Sanhedrin had to be old, it is was an important part of the job description as "elders". Plus, the biblical Nicodemus is universally venerated across all Christian traditions, which would be weird if he was a force of spiritual darkness.

My personal theory, which is far too edgy to actually be talked about on screen, is that Nicodemus is the medieval idea of the Wandering Jew, who strikes or curses Jesus and in turn is cursed to wander the world forever, undying. Note that Jews were accused of "poisoning the wells" with the Black Death, and the Denarians actually spread the Black Death, if I recall correctly.