The laws of selfdefence differ in many countries but the general principle is mostly the same. The US allows its citizens to carry a few weapons with them in order to protect themselves; however, they must have a license to carry those arms. On the other hand, Australia is against the use of weapons in selfdefence situations and is highly dependent on the police for the protection of its citizens.
Selfdefence is an act performed by an individual to protect themselves in case of murderous intent, assault and battery, rape or any other violent act being committed towards them. A plea for selfdefence can include any criminal charges or liabilities for committing a crime, causing injury to or death of another person in the situation. You may need to consult a criminal lawyer in order to get specific legal advice.