"@redcoatboom because every villain has a real human story. The best villains are relatable people. In my humble opinion."
The showrunner already has a track record for creating complicated, nuanced villains. Mona (Janel Parrish) created the A persona in order to take her power back from the people she believe made her life miserable. While the Liars may not have tortured Mona to the extent that Queen Bee Alison (Sasha Pieterse) did, they were still complicit in her bullying. Mona's actions as A were anything but justifiable, but it was still heartbreaking to hear the painful toll Alison's bullying took on Mona. In a way, Alison's cruelty helped nourish the monster that Mona became when Mona donned that black hoodie.
Charlotte (Vanessa Ray) had a different motive for torturing the Liars — one that was a whole lot thinner than Mona's, if we're being honest here — but her backstory was definitely sad. Charlotte was born male, but wanted nothing more than to present female — something that her father Kenneth (Jim Abele) was vehemently opposed to. Kenneth even locked Charlotte away in Radley because he couldn't deal with his daughter's gender identity. Charlotte spent her years in Radley almost entirely alone, and hopped up on every medication possible. Sure, her terrible life wasn't an excuse for stealing the A game from Mona, but who wouldn't get bummed out by Charlotte's story?
When King says that the Uber A reveal might be "sad," she's not saying it will be excusable — only that the motives of the villain will come from a real, human place. Don't expect to take A's side at the end of the series finale, but maybe pack a few tissues just in case Uber A's backstory gets particularly weepy.