A short BBC commentary by Professor Fawaz A Gerges:
Since the sudden appearance of the extremist Sunni Islamic State (IS), the group has seized headlines with a shocking level of blood-letting and cruelty - but can its savagery be explained...
Link:http://www.bbc.co.uk/news/world-middle-east-29123528

Here's a taster:
It is a conscious decision to terrorise enemies and impress and co-opt new recruits. IS adheres to a doctrine of total war without limits and constraints....