Muzan was the first demon, the one who wanted to conquer the sun.

It was all he ever wanted, everything he ever needed. He had so desperately tried to grasp onto the Blue Spider Lily, trying to find the precious flower for years upon years.

So who was this puny human to change goals that Muzan was adamant to achieve? A fake spouse, all to blend in with humanity and yet.. he never had the urge to kill you off once he tires of you.

Maybe it wasn't the sun he wanted to conquer, but you.