The ISIS claim to be the true Muslims and accuse other Muslims of polytheism while they are far away from the true Islam.
The terrorist group of ISIS and the Wahhabis claim to be representatives of true Islam and accuse other Muslims of polytheism. The ISIS under this pretext kills innocent people including women and children and the Wahhabis support them ideologically and financially. What the Quran tell us is to accept from whoever claims to be a Muslim. It says:
وَلَا تَقُولُوا لِمَنْ أَلْقَىٰ إِلَيْكُمُ السَّلَامَ لَسْتَ مُؤْمِنًا
"And do not say to one who gives you [a greeting of] peace "You are not a believer".[1]
Even if a people are not Muslims it does not mean that we have the right to harm them or treat them badly just because they are not Muslims. Regarding non-Muslims who do not have any ill intention against Islam and the Muslims and do not want to harm them or wage a war against them the Quran says:
فَإِنِ اعْتَزَلُوكُمْ فَلَمْ يُقَاتِلُوكُمْ وَأَلْقَوْا إِلَيْكُمُ السَّلَمَ فَمَا جَعَلَ اللَّـهُ لَكُمْ عَلَيْهِمْ سَبِيلًا
"So if they remove themselves from you and do not fight you and offer you peace, then Allah has not made for you a cause [for fighting] against them".[2]
Not only the Quran forbids us from fighting against them rather it encourages us to treat them justly and be kind to them. It says:
لَّا يَنْهَاكُمُ اللَّـهُ عَنِ الَّذِينَ لَمْ يُقَاتِلُوكُمْ فِي الدِّينِ وَلَمْ يُخْرِجُوكُم مِّن دِيَارِكُمْ أَن تَبَرُّوهُمْ وَتُقْسِطُوا إِلَيْهِمْ إِنَّ اللَّـهَ يُحِبُّ الْمُقْسِطِينَ
"Allah does not forbid you from those who do not fight you because of religion and do not expel you from your homes - from being righteous toward them and acting justly toward them. Indeed, Allah loves those who act justly."[3]
The ISIS and the Wahhabis have a distorted understanding of Islam and are far away from the teachings of Islam. They have waged a war in Syria and Iraq against Muslims and kill innocent people even women and children by exploding bombs in public areas. In fact they are very much like the Kharijites who thought to be better Muslims even than Imam Ali (as) and accused him of Kufr. Though they recited the Quran and the effects of prostration were evident on their foreheads but they fought the true successor of the Prophet (PBUH) and eventually one of them martyred the Imam when performing the dawn prayer in the mosque.
Footnotes:
[1] 4:94.
[2] 4:90.
[3] 60:8.