The idea (I refuse categorically to call this drivel a "theory") that Turalyon will become a villain has nothing to do with Turalyon, and everything to do with the Siege of Orgrimmar. After SoO, a few Horde players are certain that the next expansion will feature a "Siege of Stormwind".
What they do not understand, is that the story is not mirrored, and that they are NOT owed ANYTHING.
Turalyon is just the current ruler of Stormwind, so naturally he is the current "final boss" candidate for this delusional/coping Siege of Stormwind fan-fiction.
Back in BfA, these same people were certain that Anduin, the then-ruler of Stormwind, the "boy-king who served at the master's table and offered three lies", would become Void-corrupted/enslaved by Sylvanas and be the final boss of a "Death-corrupted Stormwind".
Seriously, does everyone here have short memory? We're going to pretend like this image wasn't everywhere exactly 5 years ago?
The desire from a subset of Horde players to raid Stormwind, as some sort of twisted "reparation" for SoO, has been going on for over 10 years and is at the root of the idea to turn Turalyon into a villain. The tune has simply switched from a Death-corrupted Stormwind with Death Anduin as the final boss (who ironically happened in SL, but obviously not in Stormwind) to a Light-influenced Turalyon in a Light-enthralled Stormwind.
There's nothing more to it.