Were there any bigger terrorists than the Nazis? Yet we didn't target individual defectors\citizens during the war. We followed the rule of law in civilized society and prosecuted them after they were captured.
It would be much better, all round, if we had the technology to grab, suspect terroists, from Syria. But we don't (as far as I am aware), currently have any such technology.
It would probably help as well. Because we could learn some useful information, by interrogating and questioning the suspect(s).
Maybe in the future, we will have super-drones, which can pluck suspects, from the ground.
On the other hand, such technology has its own worrying risks (of abuse), as well.
They could have sent in ground troops, to apprehend the terrorists. But at the moment, I think that is politically unacceptable (sending in ground troops).
Because of the public's worries about getting into another long drawn out, and disastrous war. The public, usually finds the loss of soldiers, during such operations, as very hard to swallow.
But in answer to your question. Modern technology, and the modern fields of war, are somewhat significantly changing, compared to the past.
Previously (e.g. second world war), we would have been up against a large bunch of (German army) distinctively uniformed, highly organized, conventional army.
These days (or so I've heard), we are up against, civilian like people, wearing civilian cloths. Fully mingling amongst, normal homes/houses etc. Full of mostly innocent civilians, in the very near vicinity.
If you asked the terrorist, he would probably deny any involvement with being a terrorist. They would also look exactly like, the genuine, non-combatant civilians.
They would sometimes arm themselves, and travel on the back of motorbikes and/or Toyota pick up trucks. Perform their attacks and/or leave improvised Explosive device(s). Then return to looking and acting just like the other civilians.
A sort of gorilla warfare, I guess.
These techniques (NOT necessarily that modern, as they have been used previously. E.g. The French resistance, fighting the occupying German Soldiers).
My understanding, is that it is VERY difficult to deal with enemies like that.