The most effective propaganda is simple, direct and repetitive. Isis clearly thinks that its enemies believe that they are coldhearted, psychopathic killers. And Isis uses that for their advantage. Their message is direct and clear. Don’t mess with us. We will do exactly what we said we will do.
This is pretty straightforward messaging, not the kind of propaganda that distorts facts to show a different reality.
Propaganda is all about misdirection and fulfillment. And it doesn’t matter, if you work for Apple’s advertising and marketing department, who tell people what it means to be cool and different, or you work for Isis, who tell people fight and kill people, the target is always the individual within the mass.