Bucky Barnes has long tried to atone for his sins as the Winter Soldier, and one of those sins struck Captain America's world hard. When Winter Soldier first came out into the open, he killed another ally of the Star-Spangled Avenger and a hero in his own right, Jack Munroe, a man who also once called himself Bucky in Ed Brubaker and John Paul Leon's Captain America #7 in 2005.
Jack Monroe was Bucky in the 1950s, following in the footsteps of his heroes Captain America and Bucky Barnes. He earned this role by training with a teacher named William Burnside, who created a super-soldier serum for both of them, making them the new Captain America and Bucky. The problem is the serum had side effects that made them mentally unstable. They ended up captured and put into suspended animation until Jack finally received rehabilitation that allowed him to leave a free man. He soon became allies with the real Captain America and took on the Nomad mantle. This was the same persona Steve Rogers held for a short time when he gave up being Captain America.
In "The Lonesome Death of Jack Monroe," Jack looked back on those two roles and realized he never had his own unique role as a superhero. He was always trying to be someone that he wasn't, and that included his time as a villain known as the Scourge, which was also a role someone held before him. Jack was lost, and when he went to visit Dr. Jane Foster, he learned he was dying, likely from the Super-Soldier Serum. Foster said it would also cause him to lose his mind, already damaged by his years in the '50s with William Burnside.
However, there is a reason to believe Jack's disease wasn't the reason for his downfall. Aleksander Lukin was a former Soviet general who became a successful businessman. He was also the man who found and rescued Bucky Barnes after he and Captain America fell to their apparent deaths. Lukin started the Winter Soldier program, turning Bucky into one of the world's greatest assassins while keeping the former Bucky a ghost. After Lukin had Red Skull killed, he used the Cosmic Cube to give Captain America delusions, and it is possible that the same was done to Monroe to plague him with delusions.
When Jack learned he was dying, he set out to find his daughter. He discovered that there was a drug dealer doing business near her school, and he set out as Nomad to stop him. However, none of the people he attacked and brutalized were drug dealers. They were teenagers and businessmen, all innocent of any wrongdoing.
At the end of Jack's life, he was lured to a local bar where he was met and killed by the Winter Soldier. Before he died, Jack saw the Winter Soldier as Bucky, but it is unclear whether this was a genuine revelation or the result of another delusion. His murder was intended to send a message to Captain America, and even worse, he was framed after his death and implicated in a terrorist attack, tragically destroying his reputation as well as his life.