Undo evil

Does the means justify the ends? A statement that comes out many times when coming to solve certain form of issues.  Sometimes I wonder if it justify the means to rid of one evil so it can undo many other evils. If you know one person is of evil, very much of evil and you have a choice to eliminate that one evil, would you do that?

After watching 3 seasons of smallville, beside part of the problems that caused in Smallville is due to the meteoroid shower, the other half are badly caused the Luthur family. If the hero is to put aside abit of morality and exterminate the Luthur family totally, then at least half of the problems are solved. But if that can happened once, you can continue again, and the greyness in the morality zone will just expand further.

Remind me back in the time of Romance of Three Kingdom (三国志）, the time where China is in turmoil again, which they seems to be entering every few centuries. Chinese just seems to enjoy war. Everyone just want to be the emperor, everyone just want to own a piece of the kingdom. One of the greatest strategist of that era advise the man of great charm and morality to covert and take over the lands of people whom he called brothers. Not even more when the owner of the lands are on the down side. If that great man was to take the advise of the great strategist, the entire history of the chinese might just changed. But of course, history book willl portrait him differently then.

So I just wonder, how justify is it to undo an evil by do another evil to you so as to save many other evils from happening thereafter.