The most common thread in criminals, particularly murderers, is poor impulse control and emotional volatility. People kill because they get angry, or desperate. 3/4 of people who attempt suicide will be deterred by a simple obstacle or obstruction in their way.Most murders and suicides are actually rational.
Is this even true? Criminals and depressed people may have poor impulse control, but it's a persistent problem. So the only practical implementation of your line of thought is lock all us in cells and take away our shoelaces, forever. Gun control is about limiting any action. Rolling Stone even recently had a feature that listed the victims of gun violence who were "senselessly" shot dead when they tried to mug, burgle, or rob.
any idiot...can print off their own gun
The risks you mention are implausible--security in the White House certainly does not rely on metal detectors, for example--what you are really worried about is too much freedom. While random pat-downs are normal, and body scanners have a promising future, your best hope of security is getting surveillance cameras installed in every registered living area. Not only would contraband be detectable, but women and children would be protected, since 99% of domestic violence and sexual abuse happens behind closed doors. Telescreens are a common sense modern technology: 9 out of 10 Americans surveyed agree.
As for airplanes, I think most people recognize them as a private vehicles, rather than public places, and thus not free zones. There is more cognitive dissonance with the "guns are bad" people on this issue, since they love seeing men with "military grade" assault weapons at airports, and enthusiastically support air marshals armed with "automatic pistols" with "high-capacity" magazines