This is just a mistranslation. The German news media have some truly stomach churning/eye roll worthy terms they tend to use for Domestic Violence, the most common one being "Familiendrama" which sounds like an argument over the washing up, not a mass shooting.
They are reporting the incident as being DV related because it is, it's related to a child custody case. The child and the child's mother were not harmed, which I think might be because they were not present although I'm not sure.
The people who were killed were likely social workers and other professionals working with the family, although they haven't released details yet. TBH I thought of this when I saw the thread yesterday about social workers being at risk from violent parents.
It is not that unusual for criminals in Germany to be able to get hold of guns because it has so many borders with countries which are all part of the Schengen agreement so they are unmanned. That said, it's not common either. It doesn't compare to the US for example, people aren't going around fearing violence day to day. But they do have some fairly bad stats for gun violence compared with other European countries.
The reporting in the English language media is very confusing because there are mistranslations all over the place, some of the terms used (e.g. Jugendeinrichtung) don't have a direct translation to English anyway. Essentially this is a building usually owned/run by the local social services department which may serve as a kind of youth club or hub for youth services, but can also be used as a place for meetings to happen such as the meeting which happened in this case, and sometimes they have living facilities or can be schools as well. The one closest to me is a special school for SEMH but is also a residential care home as well as running various other services. It's too far away from anything to be a drop in service but they have an office in the village centre which does have drop ins. The Jugendamt is slightly different in scope to the UK children's services/social services.