Seph also tried to kill Tifa for the fuck of it multiple times. plus she was incredibly heartbroken at her dead parents and swore violent revenge.
I'm not knocking artistic asthetic (visual it's artistic), but when it comes to this couple and how he abuses her I have one opinion:
www.youtube.com/watch?v=-K7fCQ…also Aerith can come back to life, bend the rules of destruction, and even drag people out of death. killing her doesn't count nearly as much technically.