If an user threatens another user (especially a rather new account) with getting him banned... is that OK? Or, better asked, should we do anything about it? (report or anything). Is a new (unofficial) rule needed?
No threat considers ban-able or punishable unless it's a death threat. The forum or specifically the community is against it. It is a serious offense too. But I do not believe anyone who really is planning to kill someone is going to tell it in a forum.
It's freedom of speech, let them practice it. The German board is divided into two sides. They will cool down at some point.