Anipe was not as merciless as she is now because she was once a normal girl until she became the slave of the previous Vessel of Ammit. She became the slave of the previous Vessel of Ammit because of her disfigurement that made her lower jaw appear as she has no lips. Her deformity garnered the attention of the not just the vessel but Ammit herself.
Anipe knew that she must not remain as a slave if she did not give anything to be praised. The slaves of the Vessel of Ammit are different as they fight for their lives every month. All slaves will need to either kill or greatly debilitate their opponent but the one that loses will always get fed to the current vessel's mount.