I was wondering what constitutes a provocation to which it is justifiable to use force as self defense. For example, if someone came a slapped you on the face but did not proceed hitting you (at which point it could be claimed he no longer presented threatening force to you), would it be okay to punch him and not be legally liable? Or if someone started getting in your face but not really touch you, would it be legally okay to punch them to get them out of your face? What are some examples of provocation that the law would be sympathetic with the target using physical force against?
In general, what are some guidelines, in the U.S., to know when it is okay to respond with physical force? Is ANY violence (e.g. a humiliating act like slapping somebody) enough to exempt the victim from ... beating the c**p out of or shooting the perpetrator? At which point does shooting the perpetrator becomes fair game because their actions constituted a legitimate threat?
The general rule is that using force in self-defense is justified if the person using the force reasonably believes it immediately is necessary to prevent the unlawful use of force against themselves or a third party. Deadly force is not generally justified except in response to a reasonable fear of deadly force, or to prevent certain violent crimes (like rape, kidnapping, robbery, etc.) Force is not justified in retaliation. In some states, if you're not at home (or maybe even then), you also have a duty to retreat before you can use deadly force in self-defense.
So, for your specific questions:
If they hit you once but aren't continuing to hit you, it's illegal to hit them back. You can only use force to defend yourself, not to get even.
If someone gets in your face without touching you, you might be allowed to use force, but it depends on the circumstances. You don't have to wait for someone to hit you, but you can't sucker-punch someone because you want more space in a mosh pit.
Your use of force needs to be something you reasonably believe to be necessary to stop/prevent their unlawful use of force. It also needs to be proportional to the force you're defending against; you can meet deadly force with deadly force, but shooting someone to stop them from slapping you is murder. Likewise, no one's going to believe that you really thought you had to beat the crap out of someone in order to stop them from slapping you, or that it's in any way proportionate to what they did.
Deadly force is not allowed in defense of a person unless you reasonably think deadly force is needed to prevent the unlawful use of deadly force, or to prevent one of a few crimes being committed against them. Depending on the state, it might be presumptively justified if the person is trying to forcibly break into your occupied house, car, or place of business, but that's state-dependent.
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