WMD in a BVD

It is official.  The Nigerian jihadist who attempted to blow explosives on a jet liner on Christmas Day has been officially charged with attempted murder using a Weapon of Mass Destruction, said WMD being a piece of explosive hidden in his briefs.
This term, WMD, needs some explanation, some definition.
What constitutes a mass killing?  Is it six?  600?  6000?  6,000,000?
What constitutes a Weapon of Mass Destruction?  An atomic bomb?  Poison Gas? 12 ounces of explosive?
The attack on the World Trade Center killed just under 3000 people.  What was the weapon there?  Was it the airplane?  The jet fuel?  The terrorists?
When we invaded Iraq to find weapons of mass destruction, what were we looking for?
A shoe bomber appears and travellers have to remove their shoes.
An underwear bomber appears but people don't have to remove their underwear.
Do you get my point?  I'm confused.  I don't have any answers to the questions above because I am confused. 
So are the people who are in charge of our security. Does that make you feel safe?
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