I suspect that Sharon's dealings have taken a turn for the nefarious, but I'm hoping that she will return to the light rather than become a full-on villain.
I think you've been hoodwinked. Which is fair. This is, after all, what Sharon Carter does. She becomes who the job demands. Just like how she was a sweet nurse living across the hall from Steve Rogers, for example.
Consider, Zemo's speech as the boys arrive at Madripoor. "We must remain in character at all times. Our lives depend on it."
I think Sharon is undercover, probably looking into the Power Broker and his organization. She intervenes to save Sam and Bucky from Selmy. She then does so again to rescue them from bounty hunters, and reveals herself to them even though she does not have to. If she's got nefarious motives, why? What is she gaining from it? There certainly could be some explanation offered down the road. But consider the simplest explanation, based on what we know of Sharon Carter from her previous appearances. She saw friends in danger and she took her shot, even though it probably messed up her work.
She can't tell them the truth, because of Zemo. He has contacts that could burn her, badly, if word got out. So she must now, as they did, remain in character at all times. She claims to be full of resentment and anger, all of which is probably both real to an extent and justified. She claims not to believe in the hero thing anymore. But she helps them even after she makes clear that she doesn't believe Sam can get her pardoned. She warns them to stay out of things, for their own safety. Then she helps them find Nagel and get out. She's doing what she knows she can to help, but doing it within the bounds of her cover character.