Being a proponent of ones right to bear arms and a member of the IDPA (International defensive pistol association) I've had to ask myself this very question on many occasions. Everyday that I choose to carry a firearm, I must be fully prepared to use deadly force and face scrutiny for my actions. This is the responsibility of carrying a firearm and I believe the benefits far outweight the consequenses.
Laws differ from state to state, but many states have what is called a duty to retreat law meaing that if you or anyone in your presence is being immediately threatened, you must first take action to retreat the scene and contact authorities immediately. On the opposite end of that spectrum, some states have good samaritan laws that require you to intervene in a crisis situation and assist. This does not necessarily mean that you must put your life in danger to save someones life, but if you knowingly neglect to assist a person in dire need, you could potentially be found liable.
Many factors also must take place before homicide is justifiable. Many law enforcement agencies have the "Deadly force triangle" as a departmental policy. The triangle consists of 3 different attributes that a person must show are present before proceeding with deadly force.
1)Ability- Does the purpotrator appear to have the physical and mental capacity to commit a deadly act upon yourself or another individual?2)Opportunity- Does the purpotrator have an opportunity/deadly weapon that may cause death to yourself or another person?3)Jeapordy- Is your life or someone elses life in IMMEDIATE Danger? I've heard of a 27 ft. rule... if the average person was running at you with a knife, it would take them 27 feet to reach you in the same time that it would take for you to unholster your weapon, disengage the safety, aim and fire accurately to protect yourself. That's pretty freakin far! Not many criminals give you a 27 foot warning of when they are going to attack!
Being able to process all of these factors before deciding to pull the trigger seems impossible, but the law requires that you satisfy all of these qualifications before using deadly force.
Reference:
http://www.encyclopedia.com/doc/1G1-20564013.html
I recall in a self defense class that a similar scenario to your dream was brought up. The situation involved an individual walking through a park at night and happened upon a man forceably raping a woman at knife point. The woman was obviously distressed but was not screaming for help. The concerned citizen (not knowing if the woman was so scared that she couldn't even scream) drew his firearm and yelled at the assailant to get on the ground. The perpetrator did not respond to his commands and the man fired his weapon at the purported rapist to cease the act and save the woman. The bullet struck the man in the stomach and killed him within seconds. The woman immediately screamed at the citizen that it was her husband and they were just having rough sex in the park! As memory serves me, the citizen was not charged with any criminal act, but did face a hefty wrongful death lawsuit by the wife. Moral of the story... Exhaust all of your options before proceeding with deadly force!
In your particular dream, the child may have been in danger, but not in immediate life threatening danger. You would certainly be exonerated for striking the man to stop his heineous act, but once you beat him to the point that he is no longer a threat to you or the child, then your job is done in the law's eyes. I think you would be pretty safe in this situation as it is difficult to prove that your fists are deadly weapons (unless you are highly trained in martial arts) but that's another lesson...
Sorry to write a book about this, but it is a very important issue in my life as I have been personally affected by a senseless murder of a loved one by an intruder. I've seen this first hand and would not wish my pain and suffering upon anyone.
Be safe out there!Dave