It seems very similar to American shootings. Likely a person who had some mental and life problems, and finally snapped. Media report that he lured his victims into McDonalds through a fake female account, and some of them were Turks and Kosovians, which hints at nationalistic motives, among others.
Likely decided to pick a particular date and follow Breivik's footsteps for effect or something.
In any case, it is truly horrible how one person can end lives of many just like that.