Hi Ariel,
I'm NOT a lawyer, but I can tell you that the law varies by state. Every reference I've seen notes that, so I assume it's true.
As both Rick and I noted, the key words are "I am in fear for my life." I was only half joking about engraving these words on the hilt of your lethal self-defense implements. It's a good reminder for when you're supposed to use them as they were designed.
Anyway, Wikipedia has various useful links to start from:
Right of Self Defense
Self defense in the US
Castle Doctrine (This is the "man's home is his castle" concept, and deals with when you have a legal duty to retreat and when you can legally stand your ground).
As noted, the laws vary by state, not to mention by country, but this is a place to start looking up what those laws.
Best,
F