Guns purchased by Social Security recipients?

There is no national data base of the mentally ill that I'm aware of. Yet I know people concerned that the latest restriction on the 2nd amendment defeated on Wednesday was to protect us from them. Obama wanted to identify people based on their Social Security status as needing 3rd party assistance to receive benefits so they would be flagged in the National Instant Criminal Background Check System. Is that legal for denying other rights, like speech or religion or assembly? I can't imagine that advocacy groups for the disabled didn't take this on as a very slippery slope. 

The man who robbed, raped and murdered a 21 year old OSU student last week as she left work probably wouldn't have been on that sweep of people who receive benefits, but he had recently been released from prison with a rap sheet, including sexual assault, going back to his pre-teen years. Is that a mental illness or just a very sloppy judicial system?

Obama's Executive Order squeaked through in late December probably just to irritate the gun lobby, but was first proposed in 2012 after the Sandy Hook killer was determined to be mentally ill. The problem is this rule wouldn't have caught him. He stole his mother's guns and first killed her before going to the school.
 http://www.breitbart.com/2nd-amendment/2016/12/23/obama-administration-finalizes-social-security-gun-ban/

https://obamawhitehouse.archives.gov/the-press-office/2016/01/04/fact-sheet-new-executive-actions-reduce-gun-violence-and-make-our