I realize he walked it back immediately because he realized what a stupid and potentially dangerous (for him) thing that was to say, but even if we were to take his statement as a serious suggestion, it's incredibly pointless and dumb. At what point, ever, in all of history, did a politically motivated terrorist act do good things for the cause behind the act? Blowing up the NRA conference would only give the NRA more ammo and sympathy. It would destroy all of the moral high ground that gun control activists try to claim, ruin every attempt at saying they're only advocating peace or that they're against normal people having weapons of war, and shoot holes all through any claim that they only want "common sense" gun control and nobody would ever pass laws to forcibly seize guns. The outrage and sympathy generated in favor of the NRA would drown out every attempt to push back against them for at least five years, maybe a decade.
Again, I know no one was seriously suggesting it and it was just an edgy temper tantrum that immediately got walked back, but even if it wasn't a psychotic suggestion it's just really stupid. It would be catastrophic for the position of gun control. The negative effects of supposedly 'supportive' extreme behaviors like that are quite literally why the concept of a false flag exists in history.