The answer is obvious but "mourn" is not the right word.

We FEAR that what happened in London could happen to us since our communities are relatively similar to London, but (we assume) very different from Nigeria or Yemem. 

Fear and anger cause stronger reactions than sympathy.  This is exactly the same as our reactions when the upper-middle class woman was murdered near UBC vs. when drug addicted prostitutes were murdered in downtown east side Vancouver.  News of the latter caused most of us no fear that our mothers, daughters, or friends were at risk, though of course we felt sadness for all the murdered women and their families.