Their Allowance of Killing Non-Muslims in Non-Muslim Countries and Leaving Their Homelands as Abodes of War (Dar al-Harb) to join ISIS
(To them) this is the Islamic homeland. This is similar to what is going on in European countries because of misinterpreting the concept of abode of disbelief (abode of war) and abode of peace. Scholars mentioned these two terms for their historical backround, but the contemporary fanatics misperceived this as they came to consider the abode of war to be every country where the Shariah of Islam is not enforced. They think that the people of these countries are not granted protection and that their souls and wealth are vulnerable and only may be protected by a treaty with the Muslims. This can be refuted by clarifying that the classification made by jurists was not intended to exclude those Muslims living in non-Muslim lands from Islam. It is rather a classification that serves as a basis upon which rulings can be implemented, just like the classification of the globe into political territories. The underlying objective of this classification is to apply Shar'iah rulings for non-Muslims based upon whether they are inside or outside their homeland. An example of this is the statement of Ibn Qayyem Al-Jawziyyah in his book Ahkam Ahl Al-Dhimmah that if a foundling is found in an Islamic territory it shall be deemed to be a Muslim, but if it is found in a non-Muslim territory it shall be a disbeliever. Some Shafi'i scholars held that if a foundling is found in a territory where the majority are disbelievers it shall be deemed to be a Muslim, for it may be concealing its faith. 
The tendency for takfir on the basis of land of residence relied only on one piece of evidence and neglected the others, thus giving a false notion that takfir is the only and ultimate ruling on this issue even though other juristic opinions on this matter have been given. One of these (opinions) maintains that the criterion for this issue depends on the faith adopted by the ruler: if the ruler is a Muslim the state shall be deemed to be an Islamic state, but if the ruler is a non-Muslim it shall be deemed to be a non-Islamic state. Ibn Hazm states, "Whether a territory is Islamic or non-Islamic depends on the majority of its people's faith, its ruler and owner."
The second tendency sets the criterion of whether a territory is Islamic to the land where Islam's directives are applied and Muslims feel safe to exercise their faith. An abode of Islam may turn into an abode of disbelief only under three conditions: 1- Aspects of disbelief are evident therein. 2- It shares direct borders with non-Islamic countries.
Al-Kasani mentioned, "What is meant by designating the word "dar" (abode) with Islam and Kufr (disbelief) is not Islam and disbelief per se, but the state of security or insecurity. Moreover, the relative juristic rulings are not based on Islam itself or Kufr (in this case), but on the security or insecurity."
Third trend maintains that the criterion of determining whether a specific land is an abode of Islam or Kufr is application of Islamic rulings. Ibn Qayyim Al-Jawziyyah says, “This is the opinion held by the majority of scholars. It is crystal clear that Muslims jurists made their opinions according to Fiqh al-Ma'alat (Fiqh which considers the outcomes of actions). The practices of the extremist groups in the non-Muslim countries lead to accusing Islam of terrorism. Moreover, their actions are considered the most evil tools for defaming Islam, as well as the fastest way for demonizing Islam in the hearts of non-Muslims. If these groups are reasonable, they would realize that their actions work against Islam. They do not understand Muslim jurists' views and, consequently, declare everyone reside in a non-Muslim country as a disbeliever as, for them, it is obligatory upon him to migrate to a Muslim country according to the hadith narrated by Abu Dawoud and al-Tirmidhi that the Prophet (PBUH) said, "Any Muslim who stays among the polytheists, does not belong to me." But this hadith is weak according to scholars of Hadith. Furthermore, it is not applicable to the Muslims who reside in many of peaceful non-Muslim countries. In addition, there is a multifaceted need for staying in these countries as Muslims need to learn, get medication, work, trade and preach Islam. This hadith was applicable to the period before the opening of Mecca and it was abrogated by Sahih (authentic) hadith narrated by Al-Bukhari and Muslim on the authority of Ibn ‘Abbas, may Allah be pleased with both of them, that the Prophet (PBUH) said, “There is no emigration after the conquest (of Makkah) but only Jihad [(striving and fighting in the cause of Allah) will continue] and good intention. So if you are summoned to fight, go forth."
Muslim scholars elaborated on the excellence of residing in non-Muslim countries since this may lead to guiding a non-Muslim to Islam. Furthermore, the situation nowadays is very different as Muslims can freely perform their religious rituals. How do those people understand the hadith of the Prophet (PBUH), "… and whosoever from my followers attacks my followers (indiscriminately) killing the righteous and the wicked of them, sparing not (even) those staunch in faith and fulfilling not his obligation towards them who have been given a pledge (of security), is not from me (i.e. is not my follower)". They do not understand that the terms of Dar Al-Islam, Dar Al-Kufr, DarAl-Harb and Dar Al-Mua‘hadah are now inapplicable due to the new world order and the international covenants, treaties and relations. These terms were applicable during third and fourth centuries A.H.
 Ibn Qayyem Al-Jawziyyah, Ahkam Ahl Al-Dhimmah 2/873
 Al-Muhallah 12/126. Moustafa Hamzah, 'Ann Thaqafat Al-Erhab – Qiraah Shari'ah p.233.