Bucky was arguing that killing Osborn might not make a difference in the long run, or might not have the desired outcome, basically. Someone brought up the "If you could go back in time and kill Hitler wouldn't you do it?" scenario, and Bucky replied with, "Uh...I did kill Hitler."
Given all the wackiness going on in the Marvel U at the time, it's entirely possible Bucky killed either a clone Hitler thinking it was the original, or he killed the original Hitler but Adolph and Zola already had "contingency clones" waiting in the wings, (or perhaps Hitler was resurrected somehow) so the war went on in either case.
The only other option I could think of us that Bucky killed a Hitler-clone as the Winter Soldier but thought that it was the original.
Of course, it's all just rampant speculation on my part but it'd certainly explain Bucky's point of view. "Yeah, I killed Hitler and the war didn't stop...I don't think killing Osborn is going to make things better for us in and of itself."