ISIS may be celebrating but of the nine fatalities so far, three were Kosovars and three were Turks, probably all Muslims.
The gunman seems to have lured his victims with a fake Facebook post pretending to be a Turkish girl. This raises some interesting questions: Did he actually know the victims personally? If so, and he lived in a colonised area, this might explain why most of the victims were brownskinned Muslims like himself.
Alternatively, and his reaction to being called a "wog" as he walked on the roof suggests this possibility: he may have been a self-hating brownskin who wanted to kill other brownskins and thus used a picture of a Turkish girl that would presumably be more appealing to brownskinned people.
Psychologically, this would be similar to the gay killing other gays in Orlando.
Another interpretation is that the number of non-Europeans in European cities is now so high that inevitably many of them are killed in their own attacks. For example, 30 of the 84 people killed in the Nice attack were Muslims. This shows the sheer extent of the colonisation that is taking place.