I think that they screen people that are vulnerable and convince them that this is the right thing to do. They have spoken to a few suicide bombers and some of them were mentally challenged and some were coached for weeks before they attempted to do what they did. Or should I said what they 'failed to do'.
And the 2 women that killed Kim Jung Un's brother thought it was a game. Perhaps a few of them are manipulated that way too.
Many of the ISIS fighters have also been given 'KAT' - it's a leaf they chew that makes them high. And many others have been given METH and other drugs. It's easier to do these things when you are not in the proper state of mind.
It's not an excuse for them belonging to ISIS, but it might explain how they commit these crimes and die a horrible death.