Let me explain why what Obama did was bad and why what Trump did was good.
First, the people the Social Security Administration would have added to the NICS database are not all bumbling incompetents (maybe some are). Some just need help with their finances which could be as simple as taking a check to the bank for deposit.
Second, these people would have been deprived of a Constitutional right without due process. Yes, they could appeal but the way the system was rigged they were "guilty until proven innocent". That's not the way we are supposed to do things in the USA.
And finally, I've not heard that Adam Lanza was in need of help with his finances. So why go after that demographic? All Obama did was chip a little piece away of citizens' Second Amendment rights probably so he could add that to his legacy and make gun control advocates feel good.
It was all a bunch of Bravo Sierra that needed to be undone. So attaboy President Trump! (This doesn't mean I approve of all he's done.)