I've gotta ask:
What was the closest design you ever came to a Yandere/Sadist style champion? And will we ever get one that goes 110% for those thematics?
We only have an influence on the game design aspect of the champions! There are teams that are dedicated to narrative, aka the story and lore of a champion, while we work separately on what that champion actually does in the game. For the most part they flesh out the character themselves but occasionally they have to work around the game design aspects (for example Akshan's revive was written into the story based on the mechanic we added into the game) so its possible something like the thematic you are describing appears in League, whether its through game mechanics or story writing, but I can't say for sure as I don't work on those teams.