Why is this a thread? It's not a debate, it's a specific question with a solid answer, that being: in the US, it's not automatically a crime to kill someone in the US if it's justifiable self-defense.
If you wanted this to be a debate, maybe change the title to: should killing for defense be a crime?
Its a crime in the UK, self defence in almost any form is a crime in the UK. Personally I don't think it should be as long as you used reasonable force to defend yourself.
Really? So if I'm about to get stabbed to death, I'm supposed to just do nothing?
As an almost graduated law student: excessive use of force is a form of statutory defense. So it is a crime but it's not punishable if you fulfill the following conditions:
1. The threat is immediate
2. The threat aims at a body or a good.
3. The threat aims to violate your right(s)
4. The use of force is your very last resort
5. The use of force was not proportional to the threat
6. The excessiveness of the use of force is an immediate reaction due to severe agitation caused by the threat (double causality)
If somebody is coming at me with the intent to harm, i.e. Holding a knife, a gun or some other instrument of pain, I will exercise my right to use deadly force.