One of Captain America's many enemies created by comic book writer/artist Dan Jurgens.
Now most superhero aficionados would remember Jurgens as the guy behind the now-classic The Death of Superman story, which sparked the creation of a new generation of heroes - Steel, Superboy and the Eradicator.
In the same way, Jurgens chose to create his own Captain America, and built up the storyline for almost a year, beginning in Captain America Volume 3, issue 26, all the way to issue 38.
The story began well, with a double agent in S.H.I.E.L.D., the USA's top secret government agency, discovering a valuable object in Virginia. It's a crate with the dead body of the first man to be given the Super Soldier Serum, the same one that transformed sickly Steve Rogers into the Avenger known as Captain America.
The double agent brings his find over to his masters over at Advanced Idea Mechanics, or A.I.M., who revive the "prototype" and name him Protocide. They feed him a story that Captain America is his mortal enemy and the perpertrator of his first death.
Protocide proves himself worthy of A.I.M.'s demands and is sent after a former Captain America - the U.S. Agent, whom he defeats with relative ease. He is then sent after the real deal, who fights him to a standstill before Cap is rescued by S.H.I.E.L.D.
Wanting to know more about his enemy, Cap learns that Protocide was once Clinton "Carl" McIntyre, a renegade soldier awaiting death at the firing squad, when he is given a second chance if he is willing to participate in a new scientific breakthrough. He takes the serum and is transformed into the perfect human specimen, but his heart is unable to take the strain of his new body and quits, killing him. Cap sees Protocide as the man who once could have been his partner and promises to bring him down and make him see the error of his ways.
When the two clash again, there is a poignant moment when they are forced to work together against a terrorist organisation, whom they defeat, but not before Cap has saved Protocide's life. However, that moment is lost when Protocide's brainwashing causes him to turn on Captain America despite all that has happened. Thinking Cap dead, Protocide returns to his bosses.
However, like all comic book clichés, Cap returns for the final showdown and the two clash again. When the base they're fighting in begins to crumble all around them, Protocide is finally bitten by the heroic bug and saves Cap's life, but seemingly sacrifices his own. But since this is a comic book, Protocide survives and chooses to walk away from this and presumably begins to lead a normal life.