Board Thread:Questions and Answers/@comment-1962215-20131014224458/@comment-38681266-20190526062630

Selfishness never came into the equation. It's simply that Saya was raised as a member of humanity, and bestowed with human values, while Diva was arbitrarily locked away as a monster. This difference led to Saya placing value in human lives while Diva, who was not raised humanely, doesn't regard herself as a member of humanity and so doesn't value them. Taking all this into account, the entire conflict was caused by a philosophical difference caused by Saya being conditioned to develop Pinocchio Syndrome (https://tvtropes.org/pmwiki/pmwiki.php/Main/PinocchioSyndrome), and value her 'fellow' members of humanity above her own life to the point of planning to murder-suicide herself and her sister for their sake.