Not too familiar with the "Troubles" in Northern Ireland, are you? Plenty of cruelty went down there. It was done by Catholic and Protestant guys with "red hair and freckles." Heck, I knew a fella once who's father, a constable, was gunned down in cold blood in front of his family in the name of some abstract political philosophy and a border on some map.
Extremism has had many forms over history. Nazis? Nationalist extremists. The Balkans? Same. Northern Ireland? Plenty of extremism on both sides. The Crusades? Christian extremism... I could go on.
The trick in todays world to to battle ANY philosophy that does not respect the right to exist as the overiding human right that begets all other rights. Any "ideal" that espouses the elimination of a people as one of it's primary tenents is to be rejected and fought against with ardor. The problem is that we too often lack the will to do so.
Islamic extremism is merely the latest permutation of what has been a continual pattern in human history. It is wrong and dangerous. At the same time, if we lump all those from a particular region of the world into one basket, we are committing a similar offense. In the Middle East you also have large numbers of Christians, Druse, Jews, Coptics, etc. Generalization is the mother of all misunderstanding, IMHO. It is equally important to remember that the people dancing in the streets after 9-11 are products of the political and religeous systems that they were brought up in... Systems who were enabled by severe poverty, lack of opportunity and a legacy of European colonial rule in the post WWI world.
The situation in the Middle East is not at all unlike Germany after the Treaty of Versailles. Hitler didn't have to much trouble drawing a crowd in Nuremberg, did he?
Want to end Islamic terrorism? Then we have to find a way to change the conditions that allow it to propogate. Simply locking out people of Middle Eastern descent does nothing but reinforce the idea that they are a victimized culture.
(...Jumps off of the soapbox)