In the lorebooks she would get the hive guardians rezzed and then immediately start torturing them until they were brainwashed into joining her cause. She was manipulative and evil, and even when she didn't have her memories she still continued to act like the witch queen, she'd come across as sympathetic to gain our trust, remember season of the lost?
That was xivu arath doing the torturing. Newly ressurected hive where filled in by there companions. If a ghost didn't want to ressurect a hive then the other hive killed them, some ghost had to be threatened.
I believe you’re confusing entries here.
The torture you’re describing isn’t Savathûn brainwashing the Hive Guardians, it’s Xivu Arath having captured one and attempting to interrogate them.