Well again Crossbones isn't some big compelling villain with a bunch of classic stories to his name.
Yeah they had potential to do something with him in the MCU but they swerved on that and made it simple and straightforward; he was a SHIELD agent, secretly HYDRA, had a bit of a grudge against Cap, got beat up by him, then Cap dropped a building on his face, survived and became a terrorist with now some real hatred against Cap, so much so the next time he sees him he's ready to ditch his deal of selling a bioweapon just so he could blow himself up with a bomb he apparently keeps on him any time he goes out just in the hopes he took Cap with him.
The end.
If anything it's a great contrast to Zemo who also hates Cap (and the rest of the Avengers). Rumlow took his shot the next time he saw Cap just by having a bomb on him while Zemo planned for over a year. Imagine if Steve dies
Half of Zemo's plans goes up in smoke and he has to adapt to screw with Tony.
I wonder now if Zemo was the buyer that made Crossbones go steal the bioweapon in the first place?
Overall, comic fans who are aware of Cap's stories and aren't the biggest fans of all of it have to commend the MCU guys for pivoting off that so so origin and then basically taking the best Cap stories they can that meshed with Feige's vision and endgame to make some great Marvel movies.
There was no room for Cap movies where he gets hooked on drugs, becomes WolfCap, Nomad (although a sequel of him travelling America, seeing how the country has changed since the 40s, losing faith in his country and giving up the Cap name could've been a dope and deep sequel), feels tortured over the loss of Bucky and makes Rick Jones his new Bucky, fights the Serpent Society, etc. They had some guidelines and no room for the regular ****. So they're pushed to do better and play with the genre's available (mostly channeling Brubaker's great run) and they end up unexpectedly good.