While that may be true, do you not think that they have powers that can "make things kind or decent?" Or more importantly -- less so? And without constraint, as they won't be facing reappointment any time in the future!
Next term, for example, they'll be asked to decide whether convicted abusive spouses should be permitted to carry guns. That might lead to some unkindness -- at the very least.
Only the citizenry can truly make things kind or decent, but the governmental powers were designed to be limited. The Constitution is written in vague language which can be interpreted in various ways - and it has been throughout history. The Supreme Court exists to be a kind of "referee" to deal with disagreements over interpretation and make a ruling.
So, yes, you could say that they have the power to make things kind or decent - or less so. We the people gave them that power, as we've gotten to the point where we can't seem to resolve even minor issues on our own.
We're in a litigious, sue-happy society where the lawyers profit and the judges' power to make law is expanded. And that largely happens because the legislative branch is all too often hobbled by political squabbling and grandstanding, when they're the ones who are supposed to be making the laws. They're the ones who are directly elected by the people, and the people count on them to make things kind and decent. But if they can't or won't do that, then, well, we end up having what we have.
As to your example - and all gun control/2nd Amendment cases - a lot of people blame the Supreme Court for making rulings unfavorable to gun control. But the easiest way to deal with that is for the people and their state governments to get together and repeal the 2nd Amendment. Then the Court can't make such rulings, and that power would be taken away from them. But if the people are not up to the task, then that's on them. Can't have it both ways.
If the political situation is such that there's too much gridlock and entrenchment as to make it political unfeasible to make sensible, constitutional laws or necessary amendments to the Constitution, then the Court, by default, has to take on that power. But if we have to depend on the rulings of the Supreme Court, then we're already in deep trouble.