Actually, the way it was said, it wasn't a knock against the NRA. In fact, the comment was more a knock toward Diamond Dog and showing what a beast he was.
To reverse this, think of the same situation, only with the bad guy being a white supremacist. The equivalent would be him blowing up the NAACP or something. It doesn't reflect badly on the organization, just the individual.
I was more offended by the comment in Lethal Weapon 4 where "Riggs" commented that the guy with armor over his entire body, a flamethrower and a machine gun (with built in stereo) was probably a "spokesperson for the NRA". But since Mel has gained my respect many other times, I'll let him slide with that line, since he didn't write it anyway. But I gotta admit, it was a bit funny. And that whole scene was about comedic value anyway.