This isn't an Islamist but an African thing.
Remember the Syrian rebel who was seen in some video clip to "bite" into what seemed to be an enemy's heart ? And remember the Youtube videos about Liberia, atrocities and warlords ? You could see a teenage boy holding a human heart in his hands and shouting he was going to eat it because it would make him strong.

Now the question is how this sick impulse found its way from here to there.