Why do people keep guns in their home when they know that emotionally unstable people live in the home?

Maybe it’s denial, I don’t know.
I think of Adam Lanza’s mom,
Even the Columbine parents.
Most recently, I am so sad about Reverend Rick Warren’s son. Knowing that the kid suffered from deep depression and suacidal thoughts, why did he have access to a gun?
I have to admit that my first husband was suacidal and I didn’t insist that he lose the guns and the result was as tragic as could be expected.
Is there a way to keep guns out of the hands of emotionally unstable people while respecting their privacy? I understand how people might avoid seeking mental health help if they think that they might end up on some “no gun list.” I could see how such a list could end up being used to discriminate in employment. Can you imagine? “We keep a gun on the property of our retail establishment so we need to screen people via the no gun list. Can’t have THAT kind on our property.”
Can we brainstorm a solution?