A difference exists between understanding the causal factors which led to this incident & excusing/permitting the behaviour as justifiable.
An array of factors could have led to this event.
1. Extreme religious indoctrination (concept of Jihad being one element/martyrdom)
2. Our foreign policy abroad & the actions of the west in the middle east.
3. A lack of empathy.
4. Mental illness (which it's expression took the above as it's form).
The reasoning behind the target selection (which from the sounds of it wasn't done perfectly, but that could be due to incompetence or stupidity) does appear to be politically motivated (albeit careless & capable of killing people even they don't think should be involved) - not that it excuses it (for them, or when the west has killed people abroad for it's own 'reasons')
The lack of empathy & extremist ideology (coupled with the concept of martyrdom) would have in part at least some of the reason behind the chosen method (I would assume).
Just hating the west doesn't cause this behaviour, neither does disagreeing with our foreign policy abroad or strongly opposing our actions which result in civilian causalities globally.
I don't support our activity abroad, but neither do I support this kind of barbarism in response either - both are wrong, both result in the deaths of fellow human beings & both happen for a reason (not a good one either).
Let's not be as silly to write off behaviour as "evil" & close the book on the matter.
