The problem isn't 'killing them', we've killed them many times before and others come to take the place of the ones we killed.
You can't kill ideology or religious interpretation - I don't think it has ever happened in the history of the world.
The only way to destroy terrorism of any ideology is to make it unacceptable to the society and keep it suppressed with normal law enforcement means.
There was no such level of terrorism in Iraq and Syria under Saddam or Assad - you need moderately strong government. US toppled the governments that kept terrorism at bay in both of these places - it didn't even bother to complete the job in Syria and left Iraq with a weak government which was oppressive to a large compact minority. I don't think anybody understood how weak the Iraq government is and how much it is hated in the sunni areas until ISIS took over them, but I think it demonstrates rather well that just killing off bad guys and leaving doesn't really address the problem.
In honing language - you can take a worn and chipped ebay special to an escher and spend a lot of time, but you will never get it to shave properly until you solve the real problems with the appropriate tools.
