Well again, you seem to be mixing a few mindsets, such as
“ if we truly believed it was justified, or if we thought it was the only way out of a bad situation.”
Seems like you are pointing more at war or self-defense than some crazy killing innocent people, if we are discussing the kind of mass shooting that is typical for American media to be talking about then there is no rational thought where a person could justify killing a lot of innocent civilians. Even the Jihadist must be mostly motivated by mental illness because any rational person knows those attacks will not stop any American policy, if anything it will make us pay more attention to those areas, not less.
So as I said, just the fact they conducted the mass shooting proves the individual is mentally ill to at least some degree and not normal.
Normal defined as the capacity to see the mass shooting as wrong, killing innocent people can never be seen as rational to the normal mind. Let’s remember that while these shootings get a lot of media attention, they are still incredibly rare. In a Nation of 323,000,000 people when 1 goes off the deep end in this way it is an abnormality.
While I agree we all feel anger or depression or disappointment, the normal person has coping skills, moral foundations, and respect for human life that all step in to prevent us from doing something like this, for a person to do a mass shooting they have to not have any of those mechanisms so they are clearly suffering from some degree of mental illness.
Lastly, yes we can say “I would never do that,” because it is true, normal people do not do that. 1 person out of 323,000,000 is an abnormality. Now is it possible that after a normal person says that they themselves develop a mental illness and end up doing it? Sure, it is not uncommon for a normal person to develop a mental illness, but even for those people it is rare even for someone with a mental illness to conduct a mass shooting of innocent people.
This is one of the real problems with the gun debate, firearm misuse (not self-defense) with legally held firearms by law abiding citizens is incredibly rare, the two most common types of people who use a firearms to attack others is gang members and other hard criminals using illegally owned weapons as an extension of their criminality. We can’t pass laws address them other than what we already have that is already ignored, criminals willing to kill people and face murder charges will ignore lesser gun laws.