On the changing of gun laws, I'd like to compare with Aramoana.
In 1990 NZ's gun laws were very lax, gun licences were easy to get, there was no limits on what weapons you could have beyond no fully automatics, and no real checks on what people were buying. Thus when it was discovered that David Gray had bought himself a small arsenal of weapons, there was a need to look at and re-evaluate our gun laws, and when that was done they were made far, far stricter with major limitations on ownership and importation of certain weapons, and even requiring a licence to buy ammo and primers.
While there might be something that needs looking into in this case if the weapons were obtained legally (I doubt that they were) the laws themselves are pretty good, without being draconian. I don't believe that taking everyone's semi-automatic hunting rifles off them because of one bigoted extremist is a good idea, nor would it solve any actual issue.