Why are people buying the line that this was driven primarily mental illness

[Pholus]

152. Considering the raw numbers you have a few too many nines there.

A few numbers to set this up:

Let's proceed under the assumption that the average gun owner owns guns for 20 years.

Approximately 30,000 people are killed each year by guns and 100,000 are killed or injured. Got those numbers from polifact during a fact check.

Gallup claims 47% of American households own at least one gun. Given that there 111 million households in the US, there are about 50 million households with a gun. I will assume only a single gun owner per house as a lower limit, for 50 million gun owners in the US. This is a factor of two lower than the NRA estimate of 100 million gun owners. The actual number is probably in the middle of those two but I will quote the odds as a range.

Given these numbers, the odds a given gun owner will not kill someone with their firearm over their "career" is between 98.8% to 99.4%.

But we did say "shoot" instead of kill. Since that includes both assault and accidental discharge hat increases the odds by about a factor of three. So in that case the fraction of gun owners whose gun collection will not penetrate flesh in a 20 year window is between 96% to 98%.

Now the weakness in this argument is that a fraction of gun owners are real idiots and so will injure more than one person during their gun toting career causing me to slightly underestimate the odds for the entire population. In that case, my numbers would be underestimates. I have a hard time buying that they are off by more than a factor of two however, considering that multiple gun injuries probably follow a classic binomial. So splitting the differences and being generous with the factor of two:

There is about a 99.5% a given gun owner doesn't kill someone over their lifetime.

There is about a 98.5% chance a given gun owner doesn't injure someone over their lifetime.