Also, Goober, whilst my personal belief is that it would be permissible by my view to shoot the chainsaw wielding bloke and inflict lethal force, I don't believe that it's the only option. What if he's at a distance from you, could you not attempt to get him talking? If you could get him to calm down and drop the thing without anyone being wounded or killed, that, in my opinion, would be the most positive conclusion to the series of events possible from the point at which you became involved.
Very well. You attempt to reason with the madman. Being a madman, he pays you no attention. He quickly, but not very efficiently, dispatches you with his chainsaw.
Congratulations, you have successfully avoided the issue and are now dead.

EDIT: As for the rest of you: the American colonists in the War for Independence killed many British soldiers in the fight for freedom. Were they wrong to do so?
The differences between 1776 and 2011 are so vast this is almost like comparing apples to oranges.
That's avoiding the question. But you skirt around the issue in your subsequent paragraphs, so I might hazard a guess. Am I correct in deducing that you do not view the colonists' killing of British soldiers as wrong?
But the fact remains; to take the life of another human being is wrong. There's no reason, at all, ever, that justifies murder. It's always wrong. As in your example, it could perhaps be necessary, but it's still a ****ing bad thing to do. I fail to understand how a logical argument can be constructed that results in the portrayal of murder in a positive and just light.
You seem to be equating "murder" with "taking the life of another human being". I don't believe they are the same thing. Here are some examples of taking the life of a human which I would not define as murder:
1) Defending yourself, or another person, with deadly force, in situations similar to the "madman with a chainsaw" example.
2) Carrying out the execution of a legally convicted criminal sentenced to death.
3) Killing an opponent during wartime on the field of battle.
Here's a situation. You are facing a madman with a chainsaw. He will kill you if you do nothing. You have a pistol and are an expert marksman. You have the ability to kill him before he kills you.
So, in that situation, self-defense with deadly force is necessary. By your opinion, however, it is wrong, and not morally permissible.
Do I summarize your position accurately?
Indeed you do, and of course it is wrong; it is simply less wrong than the alternative. I doubt that many men or women would say 'I wish I were in this position'; they would, after the fact, regret that it had been necessary and even occurred.
Killing is always wrong. It is somehow, however, necessary. Don't confuse the two.
The problem is that you equate "wrong" with "not morally permissible" here. Generally, we say that if an act is "not morally permissible", any person who commits such an act is an evil person, who should face the penalty of law. But here you seem to be giving him a free pass based on it being "necessary". Where do we draw the line? Would you give a pass to a rapist who claimed that a rape was "necessary" to relieve his sexual urges?
In contrast, I would draw a distinction between three categories: morally permissible, undesirable but morally justifiable, and morally impermissible. I would say that killing a person in self-defense would fall under the second category. I would also say that murder would fall under the third category.
People don't be ridiculous, morality is whatever feels good to you.
And "do what thou wilt" shall be the whole of the law?