Maybe Demacia needs an assassin?

have you ever played with or even against a Quinn?

Not officially, but she very much plays like one. The only thing keeping Quinn from functioning as a ‘true’ assassin is her lack of an escape tool, but even then her Vault can be used as a way out of fights after closing the gap with R.

Assassinations isn't really a Demacian thing. It's cowardly and goes against what Demacia stands for. Unless you're willing to break the lore we will never get a champion who is a straight up assassin.

That's why I bring up the idea of an inquisitor. Demacia is very, very strict, and while they want to be a shining pillar of all that is good they do so through an iron fist. People who agree to their way of life have a decent amount of freedom and rights, but you don't go against the King and the law. Trade out the "killing enemies from the shadows" for "spying on the citizens to protect from dissent" and you've got someone thematically similar to an assassin while also being able to fit into Demacian lore.