It was and has been proven wrong; that you actually cannot "go into an gun show and purchase a gun without showing ID".
I do agree with you that the US needs to up their game in terms of background checks... possibly modeling Japan's interviewing process. The Stoneman shooter should never have gotten a gun had this process taken place. Local background checks / interviewing of multiple people around the applicant / Character references, such as teachers should all be part of the gun-applicant, process.
To talk about Stoneman, the system failed. There were 3 police ON SITE during the course of the shooting and they did nothing to stop the shooter. Among the 30+ calls to the police warning of this living devil. Take away his guns maybe? Might of worked, but than logically speaking, whats stopping him from creating bombs and amassing the same amount of dead if not more? The point is guns don't kill people, people kill people.
The argument that we need weapons to protect yourself is very valid. Refer to the genocide that killed over ~6 million jews. With someone like trump as your president mocking other countrys, why would you not want some way to protect yourself.... the sheriffs at Stoneman who were on site during the 6 minute shooting took place, certainly did not protect those children. Why would you expect someone else to protect you, when you are the best person to protect yourself.