We do know from the WWII studies that the majority of soldiers will not deliberately aim at an individual. In facts, very few people will do so. We are taught that killing people is wrong and people generally will not intentionally do so. Modern training has over come this but the long term psychological effects might be devastating. Modern veterans suffer unprecedented suicide rates.
However, in the American Civil War, soldiers in the line of battle rarely fired at individuals. The black powder used in the weapons quickly obscured vision and soldiers were essentially pointing in the direction they thought was right and firing. It was pretty much blind. So I think in most circumstances, soldiers probably had little trouble firing. They could not know if they ever hit anyone. It was a matter of luck. Things were different in skirmisher units. A skirmisher could actually select a target. Given the WWII experience, I doubt that many actually did so.
In the British army of the 18th and early 19th century, they used the brown bess. The weapon, in the hands of a good shot, can reliably hit and individual at about 80 yards. Soldiers were not even supposed to aim. They were taught to load and fire as fast as possible. Intentionally aiming at someone was considered tantamount to murder. English officers were scandalized by the notion of American soldiers actually aiming at them during the American Revolution.