It's unclear whether the spell Maizuru put on Izutsumi is actually lethal. This World Guide chapter reveals that the spell is called "Ninpo: Babysitter" which suggests a much less ghoulish purpose, and that she in fact developed the spell for keeping track of Shuro when he was a child.
But even then, it simply ended up traumatizing him.
This chapter also features Izu, the day she was brought into the Nakamoto family as a retainer.
This one page comic is one of the few depictions of Shuro's father that exist (who was/is in a relationship with Maizuru, effectively one of his slaves) and it has always creeped me the f out. He is never portrayed in the main story.
Doing some math (Maizuru is presently 41, and Izu is only 17) she can't be much younger than 30 in this comic if she was "gifted" Izu at the time. Her relationship with Shuro's father isn't described negatively, but Maizuru only has bad things to say in the world guide comics. And connecting the dots on a drunkard slave-owner with a romantic obsession with one of them, doesn't make for a pretty picture.