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"?
Sigh, that seem to be the case. Europe really need to wake up. Muslim terrorists tried to cause trouble in one of our islands just today...