This is a topic I see quite often discussed with people...

What are your requirements for the death of a person to be justified? for me, requirements are:

1) The victim's life must be in severe danger.

2) The attacker has already injured/killed someone.

3) The attacker posseses a weapon.

4) The attacker has invaded private property and posseses a weapon.

5) The attacker pours his/her milk before putting the cereal in.

Otherwise, I don't think you should go around shooting people's heads off if they go 1 inch closer to you. Obviously limits should exist.

It's called manslaughter ...

I would justify it if a perpetrator is known to have an extensive criminal record such assault or armed assault, robbery, sexual assault, selling contraband, human trafficking, and last but not least a potential victim in a situation with a dangerously armed convict ...

Fairly simple: When your or a presumably innocent other's life is in immediate danger of lethal harm. Not before or after (though this is often much murkier in practice). Look up the Continuum of Force.

If me or my kids are in danger, I would do whatever it takes. If someone breaks into my house they are done. Honestly whether that person intends to harm us is irrelevant to me. Said person doesn't deserve the air they are breathing.

I can see that. But that's a bit extreme. Why would the burgular want to kill if they aren't caught yet? If there is a burgular, if possible, it may be safe to keep your and your children's distance from them especially if they're crazy. They are mostly after the money and not murder anyway. 

When it's an accident. I find it hard to validate taking someone else's life no matter what the circumstance. If, in self defense you accidentally kill them then that's one thing. If you intentionally kill them though... Nobody deserves that right of judgement.

I think very few people would support the right of people to commit executions in "self-defense".  It's more like defending yourself or others using potentially lethal force, not holding anything back in consideration of the attacker's well-being but rather using the most effective possible way of stopping them even though it has a good likelihood of killing them.  And then, in some cases, it does. 

It would just depend on the circumstances. But when I hear about burglars suing when they get shot it makes me want to vomit. What kind of country would put someone in jail or make them pay money for shooting a burglar who breaks into a house without "warning them" first. So stupid. The person is obviously not a safe or friendly person if they have broken into your house. Warning them may be all they need to kill you and your family. Nope not in my house. If someone breaks into my house and threatens my kids there life is worth nothing to me. I would kill them in a hartbeat and no give 2 shits about it. And if they happened to live I would tell the cops that I warned him/them first and I would stick with my story because I live in a country where I have to warn a criminal who has broken into my house and threatened the lives of me, my wife, and my kids.

Like there would be the time needed to decide this?

If alerting them to danger (you) would give them time to attack your kids or take a hostage or something, when saying nothing would let you gain an advantage you would give up by warning them, then yeah I'd expect you to do what protects your children (and I believe the law allows for that without needing to lie about it).  But if yelling a warning costs you nothing and lets a thief retreat without being riddled full of bullets, I think it is only appropriate to give that warning.