If someone is threatening your life, the use of deadly force is a natural and encouraged response.
However, legally speaking, the situation is very complex. Technically, the only time that you are allowed to defend yourself with deadly force is if there is immediate and impending danger to your life or personal well being.
Basically, the only time that you are legally allowed to use deadly force is when someone is physically trying to kill or rape you or someone in your direct care. The unofficial process of thinking that is encouraged is to think about how the jury will interpret the story; because that's how it usually goes. The very second that the individual backs down, or turns away, you no longer have any legal course to attack them.
If someone is threatening you and you deem your life is at risk you should have no hesitation to use deadly force. While it may be unpalatable to some, the fact is that when you wield a knife or gun you task the associated risks with that action. One of those risks is possible death when you wave said object around and threaten people.
If you're being attacked, you have every right to defend yourself in any way that you can. There are some instances in which you may want to just run, like if you can run and your attacker only has a knife, but if you can't run or he/she has a gun, then you should defend yourself using any means necessary.
Actually, all that's needed is the BELIEF that your life is in danger. It is not necessary for someone to actually commit violence against you, in order to use deadly force. If someone has a knife and you point a gun at them, telling then to drop it, and they fail to comply, and come towards you, you shoot them. If it happened to me, I would shoot to kill. Only sane response, in my opinion.
Bonobos use sex as a form of grooming, instead of combing their fur like other apes, they have sex, it is sort of an I like you thing, I had a gf that had the same attitude. So for a Bonobo casual sex is just like shaking hands. Though it takes a bit longer and you can catch more than just the flu, but if it doesn't do any real harm, Why Not!
"Self defense" is an excuse for evil, racist Nazi Fascist Republican Zionists to attack innocent minorities. If, in the extremely rare occasion, that someone actually does something like this to you, call the police and they will arrest the alleged suspect and put him through rehab. Never hit another human being unless he says something that hurts your feelings.
Unless it's genuinely shown as necessary you generally don't have the right to respond to threats or threatening stimuli with deadly force. The lack of immediate danger somewhat explains why but if the person attempts to harm you even after being disabled, presuming you're capable of self-defense in the first place, then I suppose such a level of force is justified.
In the case where a person does not know self-defense martial arts and can only use weaponry then I suppose there is no other alternative.