The ease of losing control is the main reason why the highest level a killer in captivity had ever reached was rank 4. The restriction of movements and lack of freedom eventually got to the killers, and they succumbed to the temptation of mutation.
The fourth reason this plan failed was because once the captives get an inkling that he wants to possess them, they usually become desperate and intentionally lose control.
Once they lose control, his years of effort to raise them will go to waste. Their mutated form will also become something he has to fight and kill, which will accelerate the fracture of his soul.
He had tried to circumvent the previous three reasons for his failure by brainwashing children from a young age, helping them become killers, and even granting them freedom.
The children didn't think that he wanted to possess them. They thought that he truly loved them. So they had no reason to despair.
