For a hypothetical place to start, let's assume it was an Arab group in support of the Palestinian movement.
This is however pure speculation as there is no proof whatsoever to indicate such.
Lets begin by removing the idea of right and wrong, that is simply a subjective thing. Each of us always thinks that ourselves and our country is right and the others are wrong.
Is it wrong to kill 5 or 10 thousand people to repay for the lives of their own countrymen and to attempt to regain their own country back which was robbed from them? I personally think so, but that is my subjective opinion.
Also, we have to remove from our minds the idea of 'civilian' vs 'military' target. No one is a true civilian because they mostly support their peoples ideals. Many Palestinian 'civilians' think it the right thing to do to hide bombs and terrorists in their own homes and to train their children to fight and die for their cause. Thus no distinction is able to be made between civilian and non-civilian. Any innocent people in this case are unfortunate 'collateral' damage.
This being said, the obvious answer is to completely obliterate the threat! No questions asked! Neutron bombs or something similar that will not leave a radioactive wasteland are preferred.
Nothing of the enemy should remain.
This would serve two purposes. First it would eliminate an immediate threat and second it will serve as a warning to others that such behaviour will not be tolerated.
No sympathy or empathy should be considered and no helping out survivors should be entertained. To serve as an effective preventative and warning the destruction should be complete.
Unless of course humanity is ready to take the next step towards growth, but that is highly unlikely.
That's my thoughts
BugEye