It's a very complex but at the same time very simple subject area - in short, ISIL are just the latest in a very long line of organisations humans have formed to perpetrate horrendous acts upon other humans. And alas if history has shown us something about such oriented organisations religion is disproportionately represented as a core part of their 'justification'.
Such groups always preport to commit their acts based upon x, y and z basis that justifies it - and religions have always been very good for giving them a flexible/user friendly rationale that can be peverted to meet their ACTUAL justification to the maleable plebs & marginalised folks they pray upon.
The actual goal both for ISIL and all those similar groups that have come before them through the centuries is simply POWER....as everything else stems from that....wealth, control, influence, etc etc etc.
So religion in it's true sense and Islam has NOTHING to do with ISIL - they're just hijacking it as a cloak/cover/vehicle to pursue their evil (sounds dramatic but is true) agenda.
Other than the first hand victims the real tragedy of their actions (and it's definitely something as an earlier poster said - that they do DELIBERATELY) is that they end up tarring/affecting the 99.999% of muslims who feel ISIL are a lunatic fringe that has nothing to do with their religion. The knock on effect is that a lot of non-muslims look at the situation too simply and assume 'All muslims are bad' and thus end up marginalising folks which CAN drive a small % of folks into the hands of the nutbag fringe.
Their 'long game' is that they want to divide the world into a 'us or them' situation (sadly and ironically much the same as the USA tried to do with their infamous "You're either with us or against us speech by GwB).
So as a staunch Agnostic (slowly sliding to Atheist) I'd urge any readers of the thread that whilst it's great to 'not let them win' by no longer travelling etc BUT it's also and more covertly exactly what they want to try and have moderate and appalled Muslims persecuted/vilified for ISIL's crimes and thus driven into their arms.
PS. Think about it for a second, it's the year 2015 and just like for the past couple of thousand years we're still killing, raping and doing untold horrific sadistic acts on our fellow man PURELY because these folks look, sound, eat or live differently to us. Un-pharking-believable.
PPS. Alas many Western countries foreign policy (think the colonial powers + the USA) is very often deep at the heart of many of the global conflict & disfunction issues. Thats in NO WAY a justification for them but the 'raping' of other countries (which has historically generally been white folks screwing over anyone NOT white) is a mess that shows no signs of being cleaned up.
ISIL need to be eliminated with extreme prejudice and I've zero doubt they will be much quicker than THEY'D like to think - but hopefully in doing this the powers that be won't end up perpetuating another vicious circle of hatred by not having a very targeted and appropriate response.