We are all subject to propaganda every day. We have little idea of the scale of its influence, since there are no control subjects.
How many young men joined the US Marines because of the influence of their friends who were already serving? Do you think they knew the truth of what they would face in Iraq and Afghanistan when they got there?
That's why US military deaths and injuries were 'hidden' from The public for so long. That's why statistics about civilian deaths in Iraq were never discussed by those in power.
It isn't a surprise to learn that ISIS are brutal and barbaric. But let's not assume we are knights in shining armour either.
The surprise should be that we are surprised! We need to ask ourselves how this can happen. These people aren't necessarily stupid, and we would be naïve to assume they are.