What can the West do? It has been debilitated by the destructive ideology of political correctness and its financial power has been sapped by the welfare state. It cannot do anything efficient. Rather it's likely that the West would wide open its door for humanitarian reasons. In a ten years time or so, we'll observe ISIS-induced street fights at home.
The men with the guns obviously believe that Allah forgives all sins, including the murder of unarmed POWs and civilians. Bad idea, but a hugely successful one.
Allah says: “Say: O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful.” [Sûrah al-Zumar: 53]
Too much of a good thing is a bad thing, including mercy.