Why is it legal to kill somone in self defense?

Answer 1

Generally, those who have killed in self-defense had no other option (other than to be killed themselves).

It's human nature to defend yourself from someone trying to harm you, and it's quite pointless of punishing someone for that.

If someone is trying to kill you, what are you going to do? Stand there and let it happen?

Answer 2

Self defense is a defense based on justification. Use as a defense depends on the circumstances and the defendant must prove four elements:

• That he was the victim of an unprovoked attack (with certain exceptions)
  
• That the threat of injury or death was imminent
  
• That the degree of the force used in self defense was reasonable
  
• That the victim had a reasonable fear of harm or death