So then the concept of individual liberty is not inconsistent with the concept that not all killing is of should be proscribed?
The concept that lethal force may be employed to mitigate an immediate and unprovoked threat of lethal force against one's person is well ensconced in American jurisprudence. We consider defending against an attacker to be acceptable, provided we did not instigate the issue, and that we are responding to a similar level of force.
If a little old lady attacks me with a stuffed animal, shooting her is out of the question.
If she draws down on me with a firearm, it's a different story.
Implicit in the situation, however, is the concept that an unprovoked (especially by physical action or illegal behaviour) attack employing potentially lethal force or with a weapon capable of serious injury or death may be responded to legally using lethal force.
That does not mean you must respond to the degree the threat is killed, but is an affirmative defense or legal justification if they are.