The people who directly did this are dead - dying themselves was part of what they were doing. It seems likely enough that there were other people involved in planning and coordiating it, but that is only an assumption.

Enormous and appalling as the result has been, all that was directly involved was perhaps a dozen people armed with knives. It's instinctive to look for a cause commensurate with the result, a target big enough to carry the load of retribution.

But maybe it does not exist, any more than it does with an earthquake.

In any case, there will be "retribution". There will more deaths, deaths of the innocent as well as the guilty. Nothing we say here can effect that one way or another.

But, let's remember, this thing was done by people who believed in the concept of retribution.

People who reject it, in all circumstances, are not the enemy. "Turn the other cheek; Love your enemies, do good to those that hurt you" - these were not the precepts of a coward or a weakling.