I think too many people forget that, for all of his alien modifications, Sephiroth IS human and IS subject to the nuances of human tragedy. Both of his biological parents were human. He was born from a human mother. Jenova was grafted into him, but Sephiroth's capacity for compassion is indicative of his true human heart. He only really goes full alien after he emotionally/psychologically rejects his own humanity. But it doesn't erase what he is on a fundamental physical and biological level.