If I had to put down a bet right now I’d say Iranian kid with a birth disability who wanted to become German, but bullied for his crippled leg and bullied by muslim students for “acting white”.
The flag in the background is the Turkish flag.
I found this picture while reading a blog-thread about ADS:
https://kosmologelei.wordpress.com/2016/07/23/ali-sonboly-selina-akim/
One of the commenters ( https://www.facebook.com/I.Bearth/ ) linked to this face-book page:
Some other interesting points made on that original blog thread: The blogger noted that the place where the perp lived is an area where many other Iranians live and it is close to the Iranian Consulate. Also not a poor area.