I was rolling my eyes opening the thread, but this is a decent spin on the theory. I just have one comment to make. While we don't know a lot about Ultimecia's motivation we can infer that she knew from Edea that SeeD would kill her some day. This may be a hard sell, because while we see Ellone gives some access to thoughts we never see unfettered access to memory. Nevertheless keep in mind that Ultimecia is a sorceress who exhibits a greater level of control over her host than Squall and co. and who inhabits Edea for a longer stretch than any Laguna adventure. Moreover, it would explain some of her behavior.
Even assuming she was already kind of evil on her own that might have pushed her over the edge to doing whatever necessary to try to avoid her fate. Time Compression may have been a response to learning that fate rather than the initial goal of her time travel. Literally trying to break time to prevent the inevitable. I mean, like you say it is a world where no one can hurt her, but more specifically it's where the person who already has killed her in the past and her future can't kill her. I'd argue Adel is probably a little more bad for the sake of being bad than Ultimecia, although that could be explained as evil for power's sake.