[Pollster1]

several other—mostly European—hostages were recovered from ISIS after officials paid ransoms for their safe return

The answer on where ISIS gets the money to kidnap, rape, and torture: European governments pay them.

[okie01]

The answer on where ISIS gets the money to kidnap, rape, and torture: European governments pay them.

A country that pays ransom puts a bull's eye on the back of every one their nationals.