Crossbones
Brock Rumlow AKA Crossbones is one of Captain America’s deadliest enemies in the comics. Quite literally, in fact, as the supervillain was (mostly) responsible for assassinating Steve Rogers in the wake of the Civil War event. While the character did appear in the movie version of the classic comic book storyline, his role was a lot more superfluous and forgettable.
It’s a shame, too, as he had a promising set-up in Captain America: The Winter Soldier. Rumlow was introduced as a crooked STRIKE agent actually working for HYDRA who, by the end of the movie, suffers terrible burns. The stage was set then for the villain to return as Crossbones in Captain America: Civil War and be a formidable thorn in Cap’s side.
As it happened, however, Crossbones was quickly dispatched (and killed) at the beginning of the movie. His destructive clash with the Avengers did kickstart the Sokovia Accords but, as the name suggests, they were created in response to the fallout of Age of Ultron. As such, there was literally no purpose to Crossbones in the film.