I've said it on the speculation page, but I think it might be better to say it here.
I have a theory on her demon form. Now I'm sure as many of you would disagree with this as Guildmaster Grovyle's comment about Cadance being the most violent character in the show, maybe more, but it's just a theory that could make for a more epic battle and offer explanation to those who were a little disappointed at how quickly Sunset Shimmer turned good.
What if, Sunset Shimmer's demon form is like Nightmare Moon? Like Nightmare Moon is a manifestation of Luna's jealousy and bitterness possessing her, what if the demon Sunset Shimmer is the manifestation of Sunset Shimmer's cruelty, manifested into a form of its own to possess her? Her inner demon, one might say. Her cruelty in its purest form. So the Elements of Harmony either destroyed her evil side when it restored her to her true form, or reverted it to its original state which she then got over out of horror at what she became. (I believe the former, but that's not to say she wasn't still horrified by her transformation, or conscious during the possession) It could be it restored her to what she was like before "turning cruel and dishonest", and seeing what her demon form was like, won't go that way again.
It is clear there is at least some sort of separation between her two forms, as she becomes more violent and evil when she becomes a demon, and this wasn't what she had intended given the apparent shock and pain she was experiencing during the transformation.
Now she did seem like the same Sunset Shimmer when she described what she did in the past, such as saying she wasn't really going to destroy the portal. But in essence she was a part of Sunset Shimmer, probably half seeing how cruel she was to begin with. I've made my comparison with Ghost Kaiba from Yu-Gi-Oh!, but perhaps a more better example would be Majin Buu from Dragonball Z.
Majin Buu is an evil demon who destroyed many planets working for Bibidi. Then he becomes friends with Hercule and decides to stop killing people and destroying cities or planets. Then he gets angry and casts off his evil side, which becomes Evil Buu. They argue over who is Majin Buu, then Evil Buu eats Majin Buu and transforms into Super Buu. He speaks from Majin Buu's point of view about a deal Goku made with him, and later says "Each time I've annihilated a planet, I've kept a piece of its energy with me". But Evil/Super Buu hadn't destroyed any planets yet, he's referring to the planets Majin Buu destroyed when he was still part of him. He also mentions fighting Gohan and Vegeta before, but it was Majin Buu who fought them. And he wasn't even "possessing" Majin Buu, Majin Buu was unconscious inside a pod inside his head. Hay, he's even referred to as Majin Buu and only adopts the name "Super Buu" after absorbing Gotenks and Piccolo.
(New edit) Also, there is a bit of similarity regarding the "separate entity, same memories" theory to Buffy the Vampire Slayer. When a person becomes a vampire, they die, then a separate entity is created to inhabit the body as the vampire. According to Buffy, "it remembers your life, but it's not you". Of course, there is a very distinct difference, such as, the original Sunset Shimmer survived (while the vampire's past self has to die), and her transformation came from within, her innermost evil brought out from her use of the crown to take over Equestria, but still.
So given all that, is the Sunset Shimmer theory that unlikely?
Of course, this doesn't mean the "demon" was "possessing" Sunset Shimmer all along, her cruelty was her own, and the true Sunset Shimmer was the main villain of the movie, and is officially in the "redeemed villains" category like Discord. Demon Sunset Shimmer only served as final antagonist for that final battle, not like Evil/Super/Kid Buu was considered main villain with 33 episodes versus Majin Buu's 22.
Another thing to note about how less evil Sunset Shimmer is than the others: She may have been cruel, manipulative, power-crazy, and dishonest, but apparently she wasn't evil enough to be turned to stone by the Elements. People say she's more evil because she tried to kill Twilight, but she was in demon form at the time. And I still think her evil side was removed from her, where this didn't happen with Discord, he chose to turn good because he didn't want to lose Fluttershy's friendship. More to the theory that Sunset Shimmer's transformation is the same as Nightmare Moon's, except the "outside force" wasn't a villain, but the crown, which seemed to "draw out her inner demon", as she wanted to obtain greater power through it and her desire for power came from her evil side and plans for taking over Equestria.
(new edit) Another thing: She turns her minions, Snips and Snails into demons as well, but only brainwashes the other teenagers as part of her plan to take over Equestria. Brainwashed teenagers, seriously? It would have been more effective if she turned them into demons after brainwashing them, so why do only Snips and Snails get a transformation? Answer: Because they have to have the evil inside them, she brought their dark sides out as demon forms of their own, just like the crown did with her.
So, anypony who expected a more epic final battle, while Sunset Shimmer herself survives and turns good, the demon Sunset Shimmer was destroyed. Same goes for anypony disappointed at how quick her redemption was. Although even the other theory should be enough for you as well.
Again, just a theory, what are your opinions on it?