The story unravels slowly.
The responsible department of the police giving of licenses for guns to Tristan van der V, had no integrity check. It was a 2 officer post where several corruption scandals have taken place in the recent decade. I assume myself because of cut backs, there is no repetitive audit system. Completely assuming responsibility and honest behavior of government officials by government officials. Normally there are no issues but government officials and police officers are human too with humans flaws. Most will give their lives for an honest country and to protect innocent people but exceptions to the rule always exist. For what ever reason that may be. Again proof that redundancy is the key to solving common problems.
And according to the news : Recent mental illness is a reason to deny a gun license in the Netherlands. This reason to deny was already existing but apparently not applied.
1# Thus we have a person with severe mental illness that could be a danger to himself and others while not willing to cope with it ( Hence the voluntary use of weapons by himself while knowing his own state of mind).
2# A failed police department.
As a side note :
Redundancy is the key to preventing and solving common problems.
Judging that everyone is evil or some people are evil from the start is a failure that will cause hate and separation(Roman Catholic ? Islam ?). Assuming people can make mistakes is accepting life in all ways.