The laws are ridiculously complicated because there's The Law, The Constitution, and The Courts which has to reconcile the laws with the constitution since they're usually not in agreement. In some states it's illegal to be the victim of a gun robbery if the robber uses your gun, sometimes unless you take some specific precautions.
And if you actually use your gun in the context of protecting yourself from someone else, the laws don't get any easier. In most states there's a very limited set of circumstances in which it's legal to kill someone, and it's not going to be obvious in the moment. Unless you work for the government, don't expect the jury to give you the benefit of the doubt. Hopefully you recorded everything.
After the fact it's often obvious that, yeah of course if this happens you need to fill out this specific form and notify this person and of course just telling the secretary doesn't count, and of course you should have a record of everything, etc etc etc. But nothing is obvious in the moment, and people have done some really stupid things because a cop told them, yeah that's good enough, because cops aren't trained in law and they're word doesn't overrule the law.