Surprise, surprise!! It was ISIS.

I guessed it was ISIS as soon as the murderous event happened. 1. A nightclub for dancing and drinking is against extremist Islamist principles. 2. The nightclub was on the European side of Istanbul. It was therefore a target for extremist Islamists. 3. Nightclub frequented by foreigners. Again not good as far as ISIS are concerned. Everyone in that nightclub was a legitimate target for death, as far as the Islamic death-cult ISIS are concerned.