The only humane thing we can do is to let as many of the victims get out of ISIS and Assad's reach as possible. We are doing far too little to support these refugees as it is. Meanwhile, ISIS would have to be crazy to send infiltrators through the refugee screening process. It is the most exhaustive immigration process in place. We should be far more concerned about tourist visas and the like. Turning refugees away is much more likely to provide ISIS with slave labor and children to indoctrinate, than it is to prevent an attack. Supporting refugees is the only smart option and the only morally permissible one.