Imagine the person of average intelligence – completely average in everything actually, average morals, average income, average relationship, average use of alcohol and other substances.
Then realise that because that is a true average, that logically 50% of the world are less intelligent, less moral, more desperate for money, in worse relationships, drink more alcohol, and take more drugs than the average person.
Then notice that it is unlikely that any one person is actually better than average in every one of these areas, and they almost certainly won’t be better than average in all of these areas for their whole life.
Testing for suitability to own a gun will never take into account all of these factors, which are recognised to be associated with irrational or anti-social behaviour (even temporarily). Gun ownership increases the chance of suicide and death by shooting for the people around the person – given the list of risk factors, this shouldn’t be surprising. What is surprising is that many still believe that these things won’t ever happen to them.