I don't find it funny.
Two wrongs, one which predominately occurred during the 12th Century, doesn't make a right.
Blowing up a school with kids because your leader was killed is also wrong.
Taking hostages for any reason is wrong. the OP was just pointing out a fact, not blaming.
That is the problem with this issue: people think that it is logical and ok because of some perception of a transgression.
Well, it's not.
In most of the developed world we have agreed to norms, treaties and agreements are the result of this. Including the Laws of Land Warfare. If you are a belligerent and think you can blow up kids, women, innocents, behead, and so on, then you obviously don't want to be a part of the rest of the world.
Those poor Australians did nothing to deserve that! Period.