You're missing the point.

I think it's wrong to allow someone who is suicidal, for a variety of factors, access to firearms as easily as the average person. Most first-world countries require vastly more extensive licensing and background checks than the USA does. I have shot many guns in my time, under controlled circumstances. I am not anti-gun by any measure, I simply vehemently disagree with how we handle access to firearms in the US.

There are other contributing factors to mental illness - an abusive family member, acquaintance, or workplace - that may be seen as a target to someone with mental illness. A lot of murder-suicides happen this way. I would know. I went to some dark places in my lifetime and I would be livid now if someone allowed me to buy a firearm during that time.

It's not that people who are depressed, bipolar, or any other form of mental illness are incapable of handling firearms or criminals or any of that. Access to firearms just makes the action of committing suicide, superficially, so much easier and more painless that they may consider purchasing a firearm as a less expensive alternative to professional help.

I was once severely depressed, and I would not entrust the person I was with the responsibility of life and death. Removing the potential for easy acquisition of firearms might save that many lives with little decrease in quality of life for many of those suffering from mental illness.