ISIS, Tunisia and Very Simple Logic

Among the 38 innocent victims of last Friday's massacre in Sousse, Tunisia, were a recently retired Scottish couple, Jim and Ann McQuire. On the BBC list of victims today, it reads:
Mr McQuire had been due to attend a royal garden party at Edinburgh on his return from Tunisia.
His friend Andrew Eadie, who was preparing to attend the event with him, said: "I can't make sense of it, I just can't understand the logic of what they have done."
With all due respect to Mr Eadie, whose feelings are quite understandable, and although I share the shock that all of us are feeling in the aftermath of this horrible atrocity, I'm afraid that there is a logic behind what happened. It is sickening, brutal and inhuman, but logic nonetheless.
ISIS might be monsters, but they are calculating monsters; very few of their outrages are committed without some underlying rationale. In the case of last Friday's slaughter, I believe I can discern some purposes for this action. This is not to deny that Seifeddine Rezgui derived sadistic pleasure from his monstrous actions; I am sure he did. However, I think that ISIS had their wider evil eye on the economic harm they could do by mass killing and, at the same time, were seeking to send a message to us, the people of Britain. And they will be drawing their own sinister conclusions from last Friday's events...
But let's talk about that later. 15.2% of Tunisia's GDP comes from Tourism, providing 473 000 jobs.  ISIS must have been counting on the adverse effect to the Tunisian economy that would result from terrorist activity, and we are seeing this happen already. Apart from a hardy minority, most tourists are leaving Tunisia as fast as they can. ISIS will be banking upon an increase in jobless, disaffected young people, many of whom will become sympathetic to ISIS. Besides this, Tunisia has shown signs of drawing closer to the West, and the jihadis do not like that. We can only hope that tourism recovers quickly in Tunisia.
Next, I believe that ISIS have their eye upon the 10th anniversary of the 7/7 bombings, and staged this attack by way of sending us - the people of Britain - a message. I am taking liberties here (as do ISIS, but in a more deadly way), but it seems to me that they are saying:
"Happy Anniversary! 10 years ago, we Jihadis showed you that we could kill you in the safety of your own capital city. Now, despite all the military might and resources you have, and despite all you throw at us, we can still kill you in large numbers where you think you are safe. You are safe nowhere".
It is up to us now to respond to that "message", and ask ourselves if we are going to be intimidated. Until ISIS is defeated, we will have to revive the spirit of the Blitz and carry on regardless. If we give in to ISIS on anything, the more they will attack.
And they have promised more attacks. As I said, they are a calculating organisation, and will be drawing their own conclusions about last Friday's atrocity. They will be doing some simple mathematics. For the loss of one man and one captured AK-47, they have traumatised Britain by the slaughter of 30 of our citizens (and eight other people from five different countries), dealing a massive blow to the Tunisian economy. They must be very pleased with their "result". What will make them even happier is that they claim to have infiltrated 4000 operatives into Europe. Now, if that is correct, and if every one of those infiltrators accounts for as many Europeans as Rezgui, that means the deaths of 152 000 people. That almost certainly won't happen, but I believe that ISIS are thinking this way. They have shown us they are capable of such things; they have cauterised consciences, impervious to normal human considerations. Lest we forget this, and if the Tunisian beach massacre was insufficient evidence of ISIS depravity, I refer you to an article in yesterday's Independent which quotes the Syrian Observatory of Human Rights as saying:
"– in an audit of the year since the declaration of the ‘caliphate’ – that Isis had executed 3,000 people in the past 12 months, 1,800 of them civilians, 86 of them women and 74 of them children."
 I never thought that I would share the opinion of David Cameron on anything, but I think him correct when he describes ISIS as an existential threat. Even if ISIS cannot destroy Britain as a nation, they will certainly be hoping to end the existence of as many of us as possible.
The only "positive" aspect of the massacre is the way ordinary Tunisians responded to the attack by sheltering fleeing European tourists, saving many lives. Some even tried to help by stoning Rezgui, and it was a Tunisian policeman who ended Rezgui's killing spree. If any further proof was needed that ordinary Muslims do not support Jihadi terrorism, then we have it now.