Um, yeah she is. All the SeeDs were dead in her time and she already ruled the world in the future. If she had never gone back to the past, Squall and the gang would never have found out about her and she could have still ruled the future and not have been killed.
That's not true. She had to go to the past to get Ellone, so she could compress time and rule the world. If she doesn't do that, then she doesn't rule everything, just her own era. Rulling her own era wasn't enough for her. Defeating Squall was just another little errand she had to run while possessing Edea.

And that's all I'm going to say here.