Going to Syria isn't necessarily joining ISIS. There are many rebel groups, some directly fighting against ISIS.
Remember how the West had a problem with...
The Taliban... then Al-Qaeda...... then ISIS.......... then ??
The names are changing but something else remains constant or is getting worse.
This will continue until weapons need to be sold. Military Industrial complex is a thing and it needs blood to lubricate its wheels.
Syrians (and Saudis) who joined the insurgency in Iraq weren't disenfranchised infact they were from the elite, often dropping out of university to go and join the "cause". Osama Bin Laden who founded AQ was born into one of the richest families in the world, had higher education and really had no hard done to him by anyone.
Europeans who join ISIS are mostly 2nd and 3rd generation "immigrants", often to mixed families and "fresh converts", they aren't particularly disenfranchised either.
Geopolitics and economics do have a role in conflicts everywhere, but the sooner you stop blaming everything on people being poor, oppressed and "western expansionism" the sooner we might actually be able to start working on a solution.
Last time I've checked Tibet is effectively poorer than Lebanon, Syria, Iraq, Iran, and Saudi Arabia, not to mention the smaller gulf states (and I'm not counting the average which is skewed because of how the wealth is distributed) and it's quite objectively oppressed but I have yet to see Tibetans blowing up restaurants in China.
Here is a formal example of what I mean:
http://i.imgur.com/r3bom1c.jpg
http://i.imgur.com/Qen8lBA.png
While some of this is hilariously old fashioned or historical period specific, it is also interesting just how many traits are consistent enough that we recognize them centuries later.
This could be astrology or tea leaf reading, but I think it merits a genuine scientific investigation. It may be anecdotal but far too many people agree the Italians are very fond of chatting, including the Italians.
All of this is to say, while it is true that Islam is dominant in countries where periodic religious warfare breaks out, it is possibly Islam is derived from collective temperament and not the other way around. It should definitely be treated seriously as a hypothesis.
That's an enormous amount of _potential_ harm even with low tech attacks like knives in crowds.