How do we know that Mordekaiser is just the villain, and not perhaps a victim who has adapted to his situation, perhaps being consumed by it? Mordekaiser's previous lore had him predate the Shadows Isles, but that isn't made clear here, so perhaps in tying him to the Isles directly, they are saying he became a part of it?
A necromantic spell, sounds like the kind that also turned the Shadow Isles into what it is, and perhaps there were two sides of the forces of the Shadow Isles? Those who were loyal to the Ruined King and remained loyal to him after the spell took affect, and perhaps those who turned against the Ruined King after the atrocity he committed?
Perhaps Mordekaiser led a coup d'état and overthrew the Ruined King with those who sided with him, and thus he now leads the Shadow Isles? Yet he is unhappy about his situation because he remembers everything of his past life, and unlike the other Shadow Isles champions who are content with their state, Mordekaiser is not content and can never be, and plots to find a way out of his miserable existence. Thus, he is not to be trusted, for while he leads the Shadow Isles, he - like the Ruined King before him - has plans for the future, which may impact and possibly even worsen the current situation of the Isles, but then again: what will you do in the face of ambition and desperation?
Mordekaiser's pain is his power, and it is also the only thing that connects him to his former life, the only thing that allows him remember what it was like to be mortal, to be human.
It would also fit the situation with the altars of the twisted treeline: both husband and wife are divided and different in how they view their situations.
But here is something you must notice about the wife on the west altar.
"For the King!"
She obviously supports the Ruined King, but there is no such dialogue for the husband of the east altar.
Perhaps the divide was that the wife supported and remained loyal to the king, even if she did not agree with his decision nor what came out of it, while the husband grew resentful with the changes and helped overthrow the king, but eventually grew consumed by his state of undeath and supports spreading it to others now.