You can't know everything though and all it takes is missing one thing for disaster to occur. Furthermore, most people don't consider their children capable of committing atrocities. Do you think Jeffrey Dahmer's or Anthony Sowell's parents thought their sons capable of that heinous shit?
But that only applies to minors. Once they're 18, they're no longer their parents responsibility. The Buffalo shooter legally purchased those weapons himself. He's responsible for his actions. How negligent his parents may have been when he was a child is irrelevant at this point.
One last thing, introducing weapons to your kids seems legally vague to me. What if you showed your kid a gun but don't keep them in the house or keep them out of reach, should you be held legally responsible if your child later commits a mass shooting with a gun he bought on his own? It won't work and there are better forms of gun control.
PS
I do support red flag laws which is something I think makes your point about warning signs very relevant. I think if a family member goes to a judge and says their relative, whether it's a child, cousin, nephew, etc, shouldn't be able to acquire a gun because of their mental health, that should be enough to keep them from buying one.