Seeing the human versions of the Mane Six having such fierce, burning hatred for each other before Twilight comes into the picture is... unsettling, especially when Fluttershy—Fluttershy—flies into a rage.
Pony!Fluttershy has been known to snap rather scarily as well, so this isn't all that surprising.
Sunset Shimmer in her demonic form was bad enough. However, when she puts on the crown and begins to transform and corrupt it, there is the horrible little detail that she is crying as she transforms. Broken Tears? Tears of Fear? Tears of Remorse? Painful Transformation? Whatever the reason, from her expression it is clear that she is not getting what she thought she would. Even worse, as the transformation works its way up her neck those tears evaporate from the heat.
The zombie-like moaning of the brainwashed Canterlot High students is more than a little creepy. "Ohhhhh..."
The Novelization raises the disturbing possibility that Sunset's pony form is also a disguise, and the demon transformation is in fact her original form. Unlikely to be canon in any form and it's only a guess Twilight had, but still a troubling idea.
Chills went down Twilight's spine. Was this Sunset Shimmer's true form? All this time she'd thought Sunset was just some pony from Canterlot, which was clearly not the case.