The game play of Kayne seems to be a continuation of his story - - It's up to the players.

From reading Kayne's lore bio and short story, it seems that his game play is in perfect cohesion with how his story unfolds. He either turns in to shadow assassin if he overwhelms Rhaast or Rhasst takes over his body. It's up to the player and how they play that decides his fate, much like the Chaos vs Order event. I really like this, it really brings the player in the world if you ask me. If this was the intention, Good Job Riot. I love it and would love to see later champions like this