Islamic extremists/terrorists justify targeting of non-believers/Taqfiris and indulging in inhumane crimes quoting Surah Al-Nisa verse 89:
Allah Almighty says:
“They (the hypocrites) wish that you reject faith as they have rejected (faith) and thus you all become equal (like one another). So do not make friends with (any of) them until they emigrate in the way of Allah. Then, if they turn back (violate their peace treaty and launch aggression against you), seize them and kill them wherever you find them (during the war), and take not (any of) them either for a friend or for a helper.” (4:89)
However, they are greatly violating the holy Quran which in succeeding verse, limits above actions during war and only against hard core/unrepentant adversaries while protecting peace loving, lawful polytheist and other of weaker denominations:
“But (do not fight) those who have allied with a people that between you and them there is a (peace) treaty, or who (losing heart) come to you in such a state that their breasts are afflicted (with this obsession) whether they should fight you or their own people. If Allah had so willed, He (strengthening their hearts) would have given them supremacy over you.
Then they would certainly have fought you. So if they keep away from you and do not fight you and send you (a message) for peace, then (in the interest of peace) Allah has left no way open for you (to launch any aggression) against them.” (4:90)
Similar views like a bar on violation of peace treaty, protection to Polytheists/weak even during war-times, are reverberated in following Ahadith:
(1) “Do not kill any child, any woman, or any elder or sick person.” (Sunan Abu Dawud)
(2) “Do not practice treachery or mutilation. (Muwatta Malik)
(3) “Do not destroy the villages and towns, do not spoil the cultivated fields and gardens, and do not slaughter the cattle.” (Sahih Bukhari; Sunan Abu Dawud)
(4) “Do not kill the monks in monasteries, and do not kill those sitting in places of worship. (Musnad Ahmad Ibn Hanbal)
(5) “Do not uproot or burn palms or cut down fruitful trees. (Al-Muwatta)
(6) “Do not wish for an encounter with the enemy; pray to Allah to grant you security; but when you [are forced to] encounter them, exercise patience.” (Sahih Muslim)
(7) “No one may punish with fire except the Lord of Fire.” (Sunan Abu Dawud).
Therefore, extremists/terrorists distort and misinterpret the true meaning and tenor of holy Quran justifying their crimes, hence, every true Muslim would be committing a crime by supporting/following these proscribed outfits/elements, posing immense danger to global peace and communal harmony.