First off, people with known mental issues (depression, ADHD or suicidal thoughts) actually can legally purchase / possess guns, silencers, machineguns and gren

First off, people with known mental issues (depression, ADHD or suicidal thoughts) actually can legally purchase / possess guns, silencers, machineguns and grenade launchers (assuming they have the proper paperwork/taxes paid). They only become a prohibited possessor when they are adjudicated mentally defective (a legal process) or have been institutionalized at a mental hospital. Secondly, issues like this aren't a 0 or 1, they are on a scale. Someone can be depressed without being suicidal and someone can be suicidal without being homicidal. Someone can have ADHD without being a danger to passengers. The FAA treats all mental issues effectively as the same thing, which results in pilots being barred from flying. Someone feeling down for a month or two after their wife dying after 30 years of marriage gets the same treatment as someone who ideates killing a plane full of passengers.