She found an icepick and murdered all her best friends.
Then she murdered her good friends. And her acquaintances. And then everypony else in Ponyville.
But they were all immortal alicorns. They could not die, so they forgave Pinkie Pie. They didn't even really care.
Since the whole town's population was alicorns, they felt sad as well. They angsted over immortality, because they would go on living forever while their friends passed away. They thought about how they would someday visit each other's graves centuries from now, while shedding a nostalgic tear.
This made them feel too sad to ever leave their homes. This made Pinkie Pie feel pretty lonely and insecure, without anypony to talk to.
So not much changed. Pinkie didn't feel any less sad than before.
She had taken over Equestria and banished Princess Celestia. The ponies now lived under her beautiful, eternal night.
But she remembered those years being shy and anxious. She was ashamed of her actions as Princess Luna. Especially that one Nightmare Night fiasco, with the spiders. That was terribly awkward.
How could anypony forgive her? She couldn't even forgive herself. Her past as a moody redeemed ex-villain haunted her. She could feel the stares of her subjects burning into her. They never talked about it, but she knew they were judging her. Bitter over the pony she used to be.
Each night she punished herself within her own dreams.
But the wounds never closed.
Nightmare Moon remained sad for the next thousand years.
Like all changelings, it parasitically fed off of other ponies' love, draining them of all vigor and emotion.
For the past few years it had fed off of Rainbow Dash's inexhaustible love for herself.
Rainbow Dash was currently stuck in the hospital with a broken wing, so that bountiful supply of love was cut off.
It had to find a new host. But where was love to be found in this harsh, cynical world known as Equestria?
The changeling nearly gave up hope, but then it detected an intense beacon of love nearby! This love radiated so fiercely, so freely, magnitudes stronger than even Rainbow Dash's narcissism. This was the mother lode!
The source was a pony named Twilight Sparkle at her favorite restaurant, the Hay Burger. She loved burgers so much. She wolfed them down, completely unconcerned with anypony witnessing her lack of table manners.
The changeling pounced on this opportunity without hesitation. It knew that its entire life had been leading up to this pivotal moment.
It used its changeling magic to reshape itself as a giant burger, diverting all of Twilight's love from the edible burgers toward the impostor. The gluttonous pony didn't notice anything at all, so she continued chowing down.
Both Twilight Sparkle and the Changeling never went hungry again.