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.