Many people claim that you shouldn't use excessive force. Many laws make the same statement. But here's the problem. How can anyone really know when the force used was excessive? Consider this scenario:
Police are called to the scene of a homicide. A man is lying dead on the sidewalk from a gunshot wound. A witness says he heard the shots, saw a woman running from the scene, and found the man lying on the ground, dead. Other witnesses identified the woman, and the police go to arrest her for murder. She claims that the man grabbed her by the arm, struck her, and fearing for her safety, she shot him.
Did she use excessive force? Would stomping on his foot have stopped him? Scratching his face? Or would these attempts have just made it more likely for him to seriously harm her? The truth is that no one knows. We don't have a do-over system whereby we can go through the same scenario with the same people, same intentions, and figure out what might have happened had different levels of force been used. In a real assault, you might have a second, at most, to decide what to do and at what level you will respond.
Perhaps the man grabbed her because he mistook her for someone else. Maybe she owed him money. Maybe she attacked him, and when he used force to stop her or defend himself, she pulled a gun and shot him. Or maybe he intended to abduct, torture, rape and ultimately kill her.
Someone might argue, "but if he was alive, then we'd know what it was really about." No, we wouldn't. If he didn't intend harm, he'd say so, and argue that she assaulted him. This is also exactly what he'd say if he had the most evil of intentions. More often than not, people who assault others and are beaten claim that they are the innocent victims.
One of the most dangerous things in the world is someone who has power, thinks he understands something, but really doesn't. Politicians, academics and armchair quarterbacks pretend to know what excessive force is or isn't, but they don't have a clue. And the truth is that no one does. All we know is how much force was brought to bear, and whether it was enough to stop the assault.
Distance = Time = Options
There is only one real life example I can think of that could clearly, rightly be called excessive or misuse of force in self defense. Not long ago person shot someone else through the entry door of his hotel room. The person at the door misread the room number, thought it was his room, and was simply trying to open the locked door. His key didn't work, of course. The person inside heard the doorknob being rattled, claimed he feared for his life, and so shot through the door, ultimately killing an innocent person. He had a right to prevent a stranger from entering his room. He had the right to use force -- even deadly force -- to ensure his safety. But he had time, he could have shouted and warned the person at the door. He also broke a fundamental rule of gun handling, which is that you never point the gun at someone -- let alone pull the trigger -- without first identifying them as a threat; it could just as easily have been a room service person at the door. When the threat is 20+ feet away, on the other side of a locked door, etc., we have choices. When the person is already on top of you, you've been hit in the head and feel like you are at risk of getting knocked out or injured, you don't have time to explore other options.
Most of the time "excessive force" is force brought to bear after the threat was over, when the threat wasn't clear, or when there were other obvious options; the police officer who shoots the fleeing or handcuffed suspect in the back, or the homeowner who kills the burglar as he's running away or uses deadly force against a subdued criminal. If I am assaulted, but knock the attacker out, and then proceed to stomp on him, kick him or beat him with a stick, I am not merely using excessive force -- I've just engaged in assault, myself. If I am attacked, frighten the attacker away, and shoot him in the back as he runs, I'm no longer defending myself -- I'm engaging in manslaughter (at the least). These aren't examples of excessive force in self defense, these are positive crimes.
In Via Potentia we teach participants to strike viciously and with 100% commitment to the most vulnerable points of the human body for the purpose of preventing or stopping an assault; our goal is to cause injury severe enough to allow our escape from the situation. We never bring injuring or lethal force to bear against someone who is subdued or fleeing, and we do not promote the application of lethal force for the purpose of protecting property. It is possible that these self defense techniques could result in the death of the attacker. Unlikely, but possible.
Duty to Retreat
Some people argue that an attacked person has a duty -- a legal or other obligation -- to retreat in the face of violence. While I acknowledge that beauracrats and others might hold this opinion or write laws to the effect, I believe that there is no ethical imperative to retreat from an unprovoked assault. We should retreat if we are in the wrong -- if we are trespassing, for example. And it may be prudent to retreat in any case. But otherwise we see no obligation to do so. In fact, I believe it is fundamentally flawed to argue that the victim is obligated to retreat, and is guilty of some criminal act should he not do so, when faced by an assault on his person, liberty or property.
In the face of violence we generally have four options; submit, flee (retreat), fight or posture. Many people simply freeze, which isn't much different from submission. Because attackers often have substantial advantages of size, weaponry, numbers, etc., rapid retreat may be the wise thing to do, but it can't be successfully argued that there is any legitimate obligation to do so. In fact, attacking may well be the most statistically-sound action when assaulted.