“...he called gun control measures in the United States “a joke” and detailed how easy it was for his brother to purchase the guns...” https://www.google.com/amp/s/au.news.yahoo.com/amphtml/chilling-instagram-note-before-family-killed-in-murder-suicide-pact-070551700.html
The real question is at any point did they seek “professional” help. Killed the family so they wouldn’t have their lives tainted by the grief of suicidal sons. That’s true mental illness if ever there was.
No gun control measures are going to prevent a citizen with a clean record from obtaining a firearm