There are a number of classes of law that are in effect. The fundamental source of law in the US is English common law. Then there are black-letter statute law (the expressed will of the legislature) and stare decisis case law (previous court decisions).
The elements of common law self-defense are; be innocent of instigation, be in reasonable fear of bodily harm, use sufficient force only to deliver oneself from evil and attempt to withdraw. Clearly legislation is being written to change these traditions in 'castle doctrines' for instance.
Defense of a third party is too complex for a 'Good Samaritan' law, more usually applied to care providers. They may be written to cover defense of another but application will be capricious and civil penalties are still in play. As there are a number of classes of law, so too are there different standards of proof and innocence. In civil court you may be required to prove innocence (rather than the state proving guilt) by a preponderance of evidence (rather than the state's burden of 'beyond reasonable doubt).
AS to rights, men are endowed by their creator with certain inalienable rights, some of which are enumerated in the Bill of Rights, specifically the Right to keep and bear arms shall not be infringed.
Either we are equal or we are not. Good people ought to be armed where they will, with wits and guns and the truth. NRA *******