1st option:

the nwo is promoting the destruction of the 'west', so that it can take its place as the dominant force asap. so, in which case the message from the deep sate cant be more clear: its okay for people of color to slaughter white westerners publicly, but its not okay for white westerners to slaughter people of color publicly. thats a taboo.

2nd option:

the attack was a false flag.. the casualties were pawns. in which case the video is a crafted psyop material. what i mean is that the shooter visited ismaili territories in his travels, where he was welcomed and had a good time, but somehow his gear is a list of serbian heroes and his music is serbian. the serbs fought the ismailis just recently, the bosnian alija izetbegovic was afaik a shia. the albanians, the mortal enemies of the serbs host the world hq of the bektashi sufi order, which are historically connected to the sect of assassins from alamut (nizari ismaili). so the serbian stuff is aimed at blackening the serbian christians. imo, the sufis are gnostics and gnosticism is the inversion of the religion. what is good in one becomes evil in the other. iow, satanists, so its an attack of a satanist order on serbian christianity, their traditional enemies.

3rd option:

they are simply trying to prevent any potential monkey see monkey do copycat attacks on mosques. and also prevent the spread of gun culture, gun use, glorification of violence, etc in face of imminent gun control. its part of their social engineering.