They do it to horrify us, the civilized people of the world and they do it to satisfy the cravings of their part of the world. The Islamic world sees themselves as the true leaders of the planet and that they will rule the world as promised by their "prophet". Every vicious and televised murder thrills their coreligionists as it should strengthen our resolve to eliminate them and their vision for the world forever.
"Moderate" Muslims do not exist.