ISIS is killing those they consider to be non-Muslims, whether or not the person they kill thinks of himself/herself as a Muslim. This has been one of the issues with the Islamic religion since its founding.
As for whether or not they follow the teachings of Islam:
- their Magazine Dabiq is constantly quoting Islamic texts.
- They are following the example of Mohammed who conquered 2/3 of the Arabian peninsula towards the end of his life
- they are following the last commands of Mohammed, which was to go out and conquer the world in the name of Islam (Jihad). This is found in Surah 9
- They are following the example by those closest to Mohammed, and had cause to understand his religion & commands the clearest - namely his companions & the following generations. These Muslims, over the next 150 years, conquered from Spain to India, and Northern Africa.
If you do not understand the difference in teachings between the Meccan Quran (where the peaceful texts are usually quoted from) and the Medinan Quran, and if you do not understand Abrogation - then you need to start looking into it for yourself. Mohammed was not a peaceful man from Medina onwards.