This proposal is about the Shadow Queen from the video game, Paper Mario: The Thousand-Year Door. A Mario villain, and there were discussions in the past regarding if she should be an PE or not. I myself think she could be an actual PE, and have some proof for why she could be one.
What's the work?
Paper Mario: The Thousand-Year Door is a RPG video game for the spin-off franchise, Paper Mario, serving as a sequel to the first game. The franchise is widely for using both Mario and RPG elements since Super Mario RPG, which was developed by Squaresoft at the the time. These elements include a turn-based combat system with a focus on action, as well as a paper-based universe.
Who is the Villain?
The Shadow Queen is a demon who remained trapped in the thousand-year door for her heinous crimes, waiting for the time she could return by possessing a perfect vessel for her. Before the events of the story, the Shadow Queen started a war when she reigned, destroying a town that is now Rogueport and causing a lot of mass murder anywhere she went. She also created the Crystal Stars along with three loyal dragons that would help ensure her reign of terror, with one of them being responsible of the murder of numerous victims including Kolorado's father, whose son appeared in the first game. For her cruelty towards the world and its inhabitants, the queen was trapped in the thousand-year door by the four legendary heroes. Even before being sealed, the queen managed to put a curse on the four heroes that only activated when they no longer had any Crystal Stars. The heroes also knew the seal was temporary and the queen would return again in the future, so they came up with a plan so the stars could be available to one who might need them.
Some time later, Beldam tricked Sir Grodus and Peach to gather the Crystal Stars in order to find the "treasure" that resides beneath Rogueport, unaware that they would free the queen to dominate the world. This led to Princess Peach being possessed by the queen, becoming Shadow Peach. With this youthful body, the Shadow Queen had plans to start where she left and reign once again over the world. The Shadow Queen proved herself to be a legitimately terrifying villain in a lighthearted series as Paper Mario, only to be later followed by Dimentio in Super Paper Mario.
Freudian Excuse/Mitigating Factors
While not much is known about the Shadow Queen herself and isn't very clear what incited her to go on a rampage against everyone on the lands, the prologue makes clear what she did that led to the destruction of Rogueport. This contrasts her from other Paper Mario and Mario & Luigi villains who had more screen time and more development on their actions and reasons to be evil. Despite her limited screen time since her battle is the last phase of the game, she makes the story told in the prologue about what she did in the past truth once we know more of her and her heartless personality in said battle.
What is presumed about her past life is that she was once a human being (possibly similar to Peach) whose physical body was destroyed by the four legendary heroes. As Shadow Peach, her first action was to destroy Sir Grodus' physical body just because the latter mere babbling of claiming himself as her boss.
The only time she showed some degree of mercy was when she saw Mario and friends, she didn't think of getting rid of them on the spot, however, she still saw they could be valuable for her to further her sinister goals. She gives Mario and company a chance of joining her. Despite Mario and team having to engage in battle with the Shadow Queen, she still had her offer opened for them and gave another chance (which wasn't optional like the first one) and if they agreed to join her, she would forgive them without problem for opposing her. However, said offer can be considered a trick similar to joining Dimentio. As shown in the image in the right, the Shadow Queen brainwashed Mario if he joined her in the Japanese version, which was changed in the US version. This means the Shadow Queen was lying all along and was going to brainwash Mario anyway and possibly the rest of his party to join her against their own wills.
Possibly her most heartless act was to consume the lives of the crowd to recover her health, the crowd couldn't do anything but scream helplessly as they were all held and absorbed by the queen's dead hands, ALL shown in battle. Her response after that? Feel relieved that she recovered from the damage she received earlier and resume with killing Mario and company. To say that they were never brought back and the player had to gather a new crowd afterwards, confirming that the Shadow Queen effectively killed them all in the spot without showing any remorse, AND if we take in considerations that the dead hands could be the hands of her previous victims, it is possible said crowd met a fate worse than death by becoming part of the Shadow Queen herself.
The only characters she didn't show any cruelty were her own servants, more specifically the Shadow Sirens, praising Beldam for her efforts at tricking anyone to gather the Crystal Stars to open the Thousand-Year Door and release her. However, she could be like this only to those she considers useful to fulfill her goals, as she didn't have any problem with trying to kill Vivian who was in Mario's side despite being a Shadow Siren herself.
From the very first game, the Paper Mario series started with a lighthearted theme just like many other regular Mario games. The franchise is well-known for a lot of nightmarish elements in past games, but most of them were just enemies with unsettling appearance, or the scary stuff was usually intended as part of the game or stage's style like Luigi's Mansion and most of the Boo centered levels. Unlike previous villains like Bowser, Cackletta or even Smithy, Paper Mario: The Thousand-Year Door introduced a lot of dark elements to the series, one of them being the infamous Toad-shaped chalk outline crime scene that was removed later in the US version. The Shadow Queen was introduced as the first genocidal villain in a Mario game, responsible of the destruction of Rougeport and the deaths of many inhabitants not only from Rogueport but possibly from other lands in the world. Being a spirit didn't stop her from cursing the four heroes and granting each one of them a fate worse than death trapped in black chests.
She would be followed by Dimentio who wanted to destroy the multiverse to recreate one universe in his own image, the Shadow Queen's true motives aside of causing chaos and death wherever she went are unknown beyond just being a power-hungered queen that desires full domination over anyone or anything. Dimentio's plan was similar in terms of being the ruler of his own dimension and getting rid of the rest of the multiverse, however he wasn't successful in killing anyone except for Mario and company who miraculously managed to escape from The Underwhere. Even in his last attempt to destroy the multiverse, he failed to do so thanks to Bleck and Tippi's sacrifices.
The Shadow Queen as shown in the prologue, is proven to be responsible of a lot of casualties, and had she been successful in killing Mario and friends, the next step was to restart her reign and plunge the world into chaos once again.
It could be said she is one of the most ruthless villains in the series and the Shadow Queen should be a PE given the content presented here. However, since the Paper Mario franchise is always trying new elements in new games, it is doubtful if she will return again.