The attacker has a LONG chain of previous sentences and a much longer police record. Started with fraud and document forgery in the 60s (possibly related to gun ownership, like the case unfolds), later also violence and illegal weapon ownership. His license to legally own a weapon was revoked because of his crimes in 1988 - you can be sure, this crime had not been plain fraud or forgery, since it takes a lot more to revoke an active permit than to be forbidden to gain a new one. He got punished for illegal weapon ownership already in 2009.
He had a long dispute with the county administration because of this weapon ownership issue as archived letters show and had a regular appointment with the county administrator. He even waited 30 minutes for the administrator. This meeting escalated into a loud dispute and chain of insults and suddenly the attacker drew his weapon, did not hesitate long, shot the administrator and then himself.
He used a big-caliber revolver, a weapon that you can not legally own in Germany as regular weapon owner, you would need a special permit for such semi-automatic weapons, even as collector. But since he was explicitly banned by his criminal record from getting a new weapon permit, you can be sure he owned it illegally.
Looks like he was banned from owning guns for a good reason...