Investigation into FUD's life has made it abundantly clear that he was a white supremacist who was savagely and unapologetically racist. He attacked this particular church because he wanted to kill black people, and because he felt doing so would start a race war that he felt white people would win. FUD did this because of racism.
A lot of people are talking about the issue and making a total muddle of it, trying to bring in all kinds of elements that are completely secondary to the central issue here. This isn't an issue of gun control. FUD wasn't using super duper military grade gear. He was using a simple handgun that he could buy easily and legally. He could have done the same thing with a hunting rifle if he had wanted to. While the gun was necessary to perform the act, virtually every gun control law proposed would do nothing to stop him. It might inconvenience him, but probably not even that.
This also has nothing to do with mental illness. We have no reason to think FUD was diagnosed with a mental illness, being treated for a mental illness, or that any sort of change in how we deal with mental illness could have accomplished anything. Last time I checked being evil and racist wasn't a medical diagnosis. Our society has challenges dealing with mental illness and there are a lot of cultural issues in that regard but it has no bearing on this case. Bringing it up only indicates that people are unwilling to face the reality that racism is the problem here, and the fact that mental illness is so often used when *white* people commit terrible crimes like this is racist in and of itself.
The problem here is hatred and scapegoating of black people. That is what we need to address, that is what helped foment this tragedy, and that needs to be where our focus is if we want to prevent such acts in future. Trying to limit guns or change our mental health policies will make a tiny difference at most because what we need (specifically in this case, what America needs) is a cultural shift. Laws will have at most a small effect compared to the enormous effect that a change in attitude will. That is what anyone talking about this situation ought to be focusing on - how terrible racism is and how we all need to do our part in ending it.