Noir (anime)/WMG

Altena gave Kirika amnesia in order to turn her into a Complete Monster.
The "different soil and water" business was a lie to throw Soldats offbase. What Altena really wanted Noir to be was the The Scourge of God by itself without her guidance. Child Kirika turned herself into an Emotionless Girl weapon in order to psychologically protect herself from what she was doing. She didn't hate any of the people she killed, she just followed orders. Period. The so called "Dark Kirika" would not kill anyone if nobody attacked her or gave her the authorized order to do so. So Altena deliberately gave her Amnesiac Dissonance so that Kirika would rediscover her emotions....so that Altena could break her more via hypnosis ritual.
 * Chloe took everything and killed to show her love to Altena, which ironically made her useless to Altena unless Altena could give Chloe a partner who was proactive after Altena was gone.

Had Mireille or Kirika directly killed Altena, some ritual would have happened to turn them into True Noir
Whether "magic" or hypnosis ingrained in both of them. However, Kirika's swandive move screwed this plan over; since Kirika falling into the lava would have meant there wouldn't be two left; so Altena abandoned the ritual to throw her into Mireille's reach.