She didn't work for Fury, SHIELD was already disbanded at that point. She worked for US intelligence (I think the CIA?) when she went rogue and stole the wings and shield. She's been on the run for at least 7 years at this point (Hulk snaps everyone back in 2023) so that's a lot of time to be bitter about getting burned for doing the right thing.
I mean Clint went full on serial killer during the snapped years and he's still free and clear post-blip, so maybe instead of assuming shitty writing, assume Sharon isn't completely on the level about what she's been up to since 2016. Like maybe she's telling Sam 'yeah get me that pardon' knowing full well she'll never get it. From a quick search it looks like the comic book version of the character has gotten into some serious shit over the years.