You are legally allowed to defend yourself or any other third party who is at risk of harm. This is whether you use your hands or a weapon to do so. However in some places you cannot use more than reasonable force to subdue the situation. meaning if it is life or death then you can kill them. However if you are able just to tackle them and pin them on the ground then you cannot pick up the gun and shoot them.

The third party act is installed so that people can protect family and loved ones but the law cannot define who the third party must be so it simply states any third party.

That is considered defense/protection of others. It depends on where you live as to whether or you would be in trouble. However, you can only use a reasonable (and the minimum) amount of force. If there is a way to defend someone without taking the life of the aggressor, than you cannot kill them. If you use to much force, you are likely to get into trouble.

So again, defense of others is good, but you cannot use a more than necessary amount of force.

Depends on the country. I know where I live you would probably also go to prison. What they do is that they start an investigation and try to determine if you used too much force. If they decide that you killing someone was without a doubt self defense then nothing will happen.

Just fyi. You would probably get better answers in the law section.

Self-defense (at least here in the US) means you can defend another person from harm. Otherwise, how would parents be able to legally protect their child? In fact, to not do so could mean that you in the eyes of the law failed to render assistance which may not land you prison, but you could be sued by the victim's family not to mention being a pariah in the eyes of the public.

It depends on the law of your state. In alabama you may use deadly force in defense of a 3rd party.

Yes. Defense of another from imminent serious bodily harm/death is legally acceptable, and treated basically the same as standard "self-defense".

That is a tough situation buddy I wouldn't know I don't work.

We all know that if someone threatens you with a gun/knife and you defend yourself, it's justified. But what if someone is being taken hostage and you shoot and/or kill the attacker? Is that justifiable? Since you're not the one being attacked, but rather, you're defending the other person, would you be arrested for shooting the attacker or not? Would that be a form of self defense?