@Utter_disgrace : more like he rebelled to free his kind, and if he had to keep one girl alive he you'd choose FL. He isn't the most extravert person and it's right after a rebellion so he must have a ton of works to do and kept reporting/had no time to visit FL to give her information.
That and the family must be very hated by most homunculus so he surely waited to be certain before claiming he'll save her (ML think FL hates him so he surely doesn't want to make her hate him more by saying false promises).
He locked her up to protect her. He just didn't though that FL's sister would kill her.