Assassination of the Winter Soldiers

Assassination of the Winter Soldiers was the result of terrorist Helmut Zemo's realization that unleashing the five brainwashed Winter Soldiers would not result in the Avengers division, but Iron Man, Captain America, and Winter Soldier joining forces.

Background
After having assassinated Howard and Maria Stark, Bucky Barnes, the original Winter Soldier, returned to the HYDRA Siberian Facility and the stolen Super Soldier Serum was used to enhance the abilities of five HYDRA assassins. However, all five individuals lost control except for Barnes during a training session, leaving HYDRA with no other option but to put them under Cryostasis Chambers.

Years later, Colonel Helmut Zemo, who lost his family to the Ultron Offensive, located Vasily Karpov, the former leader of the Winter Soldier Program, and proceeded to torture him for information about Bucky's mission and the location of the HYDRA compound. Karpov, whose loyalties remained with HYDRA, refused to talk, prompting Zemo to murder him.

Assassination
"If it's any comfort, they died in their sleep. Did you really think I wanted more of you? I'm grateful to them though. They brought you here."

- Helmut Zemo to Iron Man, Captain America, and Winter Soldier

Using the Winter Soldier Book and knowledge he obtained from the Winter Soldier, Zemo reached the facility. As he explored the abandoned place, he obtained surveillance footage of Bucky executing the Starks. Reaching the main chamber, he found the five candidates from the Winter Soldier in Cryostasis Chambers, including Josef. Knowing that the Avengers were aware of his identity and were also heading to the facility in an effort to stop him, Zemo realized that using the Winter Soldiers would force the Avengers to join forces, instead of them destroying each other, which was Zemo's ultimate plan. He followed through with his plan by executing each soldier with a bullet to the head.

Aftermath
Hours later, Captain America, Iron Man, and Winter Soldier arrived at the facility with the belief that the Winter Soldiers were activated. Zemo then greeted the three as they entered the colossal chamber, and further explained that he never intended to use the Winter Soldiers, but wanted to destroy the Avengers from the inside. Captain America discerned that Zemo was in fact a native Sokovian, which Zemo confirmed and revealed that he lost everyone he loved during the Battle of Sokovia and confessed that he orchestrated behind the bombing attack in Vienna as well as the Avengers Civil War. He then used surveillance footage to reveal to Iron Man that the Winter Soldier was in fact behind his parent's assassination in 1991. This led to a brutal fight between the three heroes.