Despite being identical, Premiere and Tia are actually polar opposites and-even worse-enemies. After learning of the sisters' unprecedented potential for world destruction, Kirito and the gang are forced to put their quest on hold in fear of losing their beloved Ainground and its NPCs. Meanwhile, Genesis, seeking to tap into the twin goddess's powers, challenges Kirito to a battle for Premiere! Which black swordsman will emerge victorious?!
Now that Premiere and Tia have met the extent of Genensis's influence can be seen as Tia tries to convince Premiere that the best choice of action is to complete the quest and destroy the world. Once destroyed they can remake it into a paradise without any annoying humans (with the exception of Genesis I bet.) Two NPCs who are both priestesses and goddesses as the quest needs them to be have learned from distinctively different masters and have become like day and night even if they are identical in appearance. Now Kirito must do whatever he has to do to stop Genesis before he murders all of the NPCs in the world (if you don't remember from previous volumes when an NPC is killed they don't respawn but are replaced.) Considering how advanced the AI for NPCs is in the game I am not surprised Kirito sees them as worthy of saving.