Maybe "Islamist" or "Muslim fundamentalist" would be better choice of words. Points at their declared reason for violence while not dragging all Muslim along. Unfortunately since the media always try to avoid the M and I word, he'll probably just be labelled "Russian teenager" or just "teenager" in the news.
Right now the mainstream media in the western world is heavily pro-Muslim, and rabidly anti-Russia. So the phrasing suits them. I just have one doubt. If some French or Italian born Muslim commits a terrorist attack in Europe, will they give the news as "French teenager blows up at the pub, killing two dozen"?
Always in the news talking about the attack, always talking about the country from which Muslims came. After the terrorist attack in St. Petersburg media also reported that terrorist is a Muslim from Tajikistan.
Could you name your media? To my knowledge, religion is NEVER menntioned as a factor and nationality only in cases, that attack occured before said person gained permanent residence.
I can vividly recall example when four "swedish youths" gangraped and maimed woman on hotel boat in Baltic sea. Turns out, there were four Somalians aged thirty in the mugshots. They had swedish passports, though.
When you hear about ISIS "foreign fighters", you also hear Belgian, English, French, Swedish all the time - logically as those countries host largest immigrant muslim diaspora in Europe. Yet, those of native european origin are in single digits...
