A very small minority of the fundamentalists from all the violent religions that talk literally about endless torture in hellfire (I notice that major sects of Christianity over the last few decades have stopped mentioning hell) will continue to commit atrocities similar to those we witnessed in Western Europe.
These tormented minds are so mentally brutalised, I presume by growing up in authoritarian households of marginalised communities, that they want a quick way out of their miseries, ie fast forward to reach endless bliss in fictitious paradise.
The IS warriors proceed with violence when they notice that vacuous institutions like the United Nations and the complicit silent spectators (sheeple) in western countries are doing nothing to stop atrocities being committed by Syrian, Iraqi, American, British, Russian and other governments.