By succumbing to the darkness in their hearts when they were human. Without a heart, they can only act based on their memories of when they were human. The Organization members were a special case, in that they were strong enough to keep those memories and appear human.

[She pauses, and then gusts out a sigh, her bangs lifting from her face.]

Of course, there are even more special cases than that. But...do you want to keep hearing about them?