Self defense is not a legitimate reason or valid purpose of owning a firearm or obtaining a firearm's licence.

Different states have slightly different laws.

In SA the laws changed that a person may use reasonable force that they personally feel is needed to protect themselves. From an earlier "common man in the street type belief" to their belief personally. After this the cops tried to charge three persons who shot a person in self defence and lost all three cases. I assume these laws and common law verdicts are still in force. So in SA I would have no problem in shooting someone if I personally felt my life was in danger and believed shooting them was the necessary way to protect my own life, family and/or property.

Regarding locking up in a safe firearms and ammunition. It is common to keep these unlocked returning or going on a trip, cleaning or working on them etc. Also it is in your control if you are home at the time.

No legal advice is given above or intended to be given.