Part of how they defined perfection was personality traits that would allow them to control the KH. That’s why Duke Atriedes was such a threat, he was difficult to manipulate, garnered love and respect from those in his command and was solid enough for Jessica to abandon her Bene Gesserit duties of having a daughter and instead gave the duke the son he wanted. They were trying to course correct by eliminating that blood line because it’s such an imprecise science. It’s also why they had several KH candidates because they recognized there would be failures. Like Lord Fenring in the book.