In the Michigan case the parents screwed up so badly that criminal liability is appropriate. The Highland Park shooter is less clear, but that was also a bad mistake. I have no problem with holding people accountable in clear circumstances but the line will be blurry for quite some time.
I would give my daughter access today if it were legal, but my son (age 13, has Coffin-Siris syndrome) may not get access at 18 or at 21. He is super friendly, empathetic, and would never be the type to shoot up a school, but I do not know how badly his intellectual disabilities affect his understanding of the consequences of pulling the trigger. Parental judgment cuts both ways.
I know people who kept loaded guns in nightstands with young but typically developing children. It scared the s*** out of me. I know some of the dumb stuff I saw with kids when ai was one.