Are you under the impression that people deemed mentally defective by the courts are now able to buy firearms? That's not the case at all, even after Trump signed this bill.
It doesn't matter in my cases due to state laws, but having seen some of the worst of what can happen to people mentally speaking, I have no tolerance. Guns are extremely dangerous things that should be respected. Unlike Republicans, I don't bother with some delusion that the Constitution is some infallible thing to (selectively) revere. While I don't believe guns should be banned, I don't believe they're something that people should have the
right to. It should be a privilege, and one that's extremely easy to lose.
If you can't manage your finances, for instance, you shouldn't have a gun. You might not be violent because of that, but you're obviously cognitively lacking. Guns don't require an inherently violent individual to be dangerous. They just require somebody to make a mistake in a judgement call - a mistake that can easily end up fatal.
Make no mistake, the law wasn't perfect. The Republicans aren't going to fix the problems though. To them, fixed is no regulation.