WARNING: The following contains spoilers for The Falcon and the Winter Soldier Episode 6, "One World, One People," now streaming on Disney+.
Sharon Carter had been absent from the Marvel Cinematic Universe since abruptly going on the run before the finale of 2016's Captain America: Civil War, not even being mentioned, save her photograph appearing in a display of vanished heroes, in Avengers: Endgame. Fans were frustrated by this and by how the MCU handled Sharon's character overall. They also saw her announced return in the Disney+ series The Falcon and the Winter Soldier as a narrative redemption. Unfortunately, it wasn't as much of a victory as people hoped.
When Sharon first appeared in Captain America: The Winter Soldier she was posing as Steve's neighbor to keep an eye on him. She then proceeded to help fight the Hydra uprising within S.H.I.E.L.D., and after the organization's fall, she joined the CIA. Sharon then helped in the actual Civil War by stealing Steve's shield and Sam's wings from custody so they could fight the good fight. She then went on the run and wasn't seen again until TFATWS.
The revelation that Sharon, unlike other Avengers and associates, hadn't been pardoned and was still considered a fugitive even after the Blip was confusing to many fans. Now that the whole season has aired, it seems like her fugitive status was mostly a plot contrivance to put her in Madripoor, which was in turn a convenient and easy reason to show her compromising her previously solid morals.
Her initial cynicism and change in personality was remarked upon by Bucky, which emphasizes how marked it was, since Bucky had known Sharon for all of a day prior to their reunion in Madripoor. However, the disconnect of returning to the world post-Blip and the frustration of being a fugitive after devoting her life to a government and society that's now abandoned her could explain this.
Speculation soon arose that Sharon was actually the mysterious figure known as the "Power Broker," someone who enabled Karli Morgenthau and her Flag Smashers. Sure, she involved herself in Sam and Bucky's fight, but she also had suspicious criminal connections in Madripoor's underworld, and, once again, she was acting very unlike herself.
This remained unconfirmed until Sharon and Karli's confrontation in the finale episode. Sharon mentioned that she had provided Karli and co. with the super-soldier serum, and then Batroc declared that Sharon was the Power Broker. After her pardon by the US government and reinstatement to the CIA, she's shown talking on the phone to someone about how she'll have access to new secrets she can sell.
If Sharon really is the Power Broker, it's a huge detriment to her character. Her motivations are never fully explained, but the implication is that she's bitter and angry that she wasn't pardoned, so she became a supervillain. In a universe where the majority of "Team Cap" continued to fight the good fight despite being fugitives and then were pardoned at some unclear point following Infinity War, making Sharon the one outlier despite the fact that she initially helped the others is, at the very least, an insult to a character who has always deserved better treatment from the writers.
Sharon's situation is even worse when you factor in the metatextual reasons for it and the implications of it. Bringing Sharon back after several years just to reveal at the last minute that she's become a nefarious evil figure reduces her to a dramatic plot twist. There's some precedent in the comics for Sharon acting as a villain, but it's always when she's brainwashed or controlled by someone evil; Sharon in her right mind is a heroic figure. If the MCU made her an actual villain, they'd be throwing a long-established character from the comics out for shock value. Much like what happened to Trish in Jessica Jones, it would be a confusing reduction of an interesting and formerly heroic character. This is especially bad because if Sharon had been included in Infinity War or Endgame -- or any of the other MCU projects between Civil War and TFATWS -- she likely would have been pardoned when the other characters were and therefore would have had no reason to become a villain.
It's possible that Sharon isn't the Power Broker, but is instead working for and/or indebted to the Power Broker; others speculate that Sharon is working for Nick Fury. There's also the possibility that, in keeping with various comics storylines, Sharon has been brainwashed. Another popular theory is that Sharon isn't Sharon at all, but a Skrull posing as Sharon for whatever reason, which would tie in with the upcoming Secret Invasion series. While these theories could lead to Sharon unnecessarily being treated as a damsel, they would still allow her to emerge as the hero she deserves to be, which her return in TFATWS denied her.
Directed by Kari Skogland, The Falcon and the Winter Soldier stars Anthony Mackie, Sebastian Stan, Emily VanCamp, Wyatt Russell, Noah Mills, Carl Lumbly and Daniel Brühl. The series is available on Disney+.