The mainstream libertarian view is simple common sense (as it is on most issues, really). A clear and present threat (either deliberate, such as someone brandishing a weapon, or negligent, such as someone driving while seriously impaired) justifies such action as may be necessary to disarm the threat.
Determining what constitutes a "threat" does call for some thought. For instance a neighbor with a gun is not (generally -- somone who has given indications of mental instability is another question) a threat, because a gun is a targeted weapon. A neighbor with a garage full of explosives is a threat, because explosives are dangerous to anyone within the blast radius.
The initiation of force is immoral. Always.
However, if it is clear that an individual intends to initiate force against you, it is not incumbent upon you to wait until he strikes to utilize force in your own defense.
You may morally act in your own defense prior to being struck.
An analogy... If a mugger just shot the man next to you, then proceeds to raise a gun in your direction, it can clearly be assumed that the mugger has a hostile intent. This would morally entitle the use of force in your own defense.
I think the analogy applies quite readily here.