Armed Self-Defense and Lethal Force
Needless to say a knife should be employed for the purpose of self-defense only when no lesser level of force will do.  You should use lethal force only when it is so important to stop the attacker that whether or not they are killed or maimed as a result of being stopped is a secondary consideration    Typically the threat of imminent death or grave bodily injury to yourself or another innocent person is the only justification for such an action  (consult your local laws and statutes for more details.)

If you intend to carry a knife or any other lethal weapon for the purpose of self defense I strongly suggest taking a course from a qualified professional in the field.  Such instruction will include not merely the use fo your weapon but also the mental preparation needed to survive a lethal-force encounter, when it is or is not appropriate to use lethal force and the legal ramifications of the use of lethal force in self defense.

You should also take care to carry your weapon in accordance with local laws and statutes.  While it may indeed be better to be 'tried by 12 than carried by six' insuring that you are in compliance with the law will make a diffecult and horrible situation (the legal aftermath of using lethal force for self-defense) a good bit less difficult and horrible.