It’s a question some fans have been asking themselves ever since it was revealed in Captain America: The Winter Soldier that Hydra had a hit put on him. We know that Bucky was Hydra’s puppet assassin for decades.
Straight from the set of Captain America: Civil War, screenwriters Stephen McFeely, who also wrote The Winter Soldier, and Christopher Markus weighed in on the conundrum to CinemaBlend.
According to McFeely:
It is in play that Hydra killed Howard Stark. I think we can say that. I think that’s all we know for sure, and Bucky killed a lot of people. I mean, that’s perhaps more important, is like, when Bucky wakes up, or gets some semblance of his humanity back -- if he were to get some semblance of his humanity back -- how does he feel about the long list of kills on his record, you know?
The actor who portrays Bucky Barnes, Sebastian Stan, was a bit more cryptic when approaching the question, he said:
I feel like it’s pretty given if you know the comics. Bucky’s in a place where he’s not very stable or [in a] healthy environment in his head. So he could easily go either way.
This presumably is referring to Howard Stark’s death in the comics by way of a car crash arranged by his business competitors from the Roxxon Oil Company. Markus went on to further describe the inner turmoil Bucky faces now that he’s no longer the brainwashed Winter Soldier.
It was bad enough when Cap woke up after 70 years in the ice, having done nothing. [Bucky] woke up after 70 years having murdered 100 of the best people of the last century. Do you want to go on? Do you want to be free? Do you want to put a bullet in your head? What is it?
And that’s the fun stuff, that’s the rich stuff we’re trying to dick around in, and Sebastian is crushing it.
We have yet to figure out if the Winter Soldier was the one who murdered Howard Stark, but it could exacerbate the growing conflict by giving Tony Stark incentive for hating Bucky who belongs to Team Cap. We'll just have to wait and see as the ever-expanding Marvel Cinematic Universe continues in Captain America: Civil War.