Have you ever heard of 'never use a cannon to kill a fly'? I wouldn't call Bucky their 'best man'. Their deadliest most dangerous weapon, sure. But you don't need that to kill Howard. Another assassin, or even a team of such, could have killed Howard just as dead. Cap couldn't rule out that it was Bucky, but he had no real reason to believe it was him, either. Not unless HYDRA no other hitmen.