It would be better to ask who and why created the Islamic state, and if you do not know, I would suggest you do some research.
Foreign policies from the west caused the rise of Isil due to the US invasion of Iraq, any yet The US supported such a group, same with Al-qaeda that was born from US policies, they gave birth to it and many more groups such as them.
Any bombing of Syria at all is illegal, they have no right to be there, the killing of innocents is not justified, France did send bombers just recently after the Nice attacks, but France are just as bad with sticking their noses into all and sundry.
Just keep out and away, and the West will be a lot better off imo.
Everyone isn't at equality. If we want to go to some country and destroy it, we can perfectly, because we're the stronger and surely the only country in the world that can be at equality with the USA. If we went there, there's a reason that I don't know. France did not helped the rise of ISIS as the USA, so they just fight back. But that's totally useless. What we need is 500 000 soldiers and 2000 tanks, with half French and half Russian. Then the time of ISIS will be a matter of hours. We also need to exterminate, litterally, with public executions, all the other "rebel" groups. Final task would be to help Assad rebuild the country by the French enterprises. A few nuclear centrals will be helpful to them.