That's right, nothing is happening. It's over. There's not more to discover, the dude will stand trial for manslaughter and probably be put in a mental institution instead of jail. There is no mystery here except if he really yelled Allahu Akbar so the Islamophobes can have another go at being outraged or something else entirely, so everyone can go "Wtf is with these mental people?"
Seriously, this is not news. A dude freaked out. It happens once a day in Germany, we don't report every one of them.