What you say is correct as it applies to the reasoning of Swedish authorities. I can't speak to anyone else's.
Banning viewing and possession of child pornography is in part a moral law, but mainly it is to limit the demand for child pornography. There's a lot of interesting history regarding the legality of child pornography in Sweden and Denmark, but it's off topic here.
Okay, let me bring it back to the topic and explain my reasoning here.
We are talking about New Zealand banning the viewing, downloading and distribution of the mosque killing videos that appear to have been made as though he were LARPing an FPS, such as Doom or Wolfenstein.
There are a few reasons why New Zealand might do this. I agree with one or two reasons and not with others.
1) The video re-victimizes the families and survivors (I think this may even be the official reason given).
2) The video may lead to copycat attacks (someone watching it may want to do what they see in the video).
3) This is part of an increasing trend of, for want of a better term, snuff movies such as those produced by Mexican cartels, ISIS head choppers and psychopaths reflecting a demand for such content.
I think that the demand/market for these types of videos needs to be taken as seriously as that of child pornography because of the demonstrable harms that exist in the making of these videos.
Of course, I understand the arguments that "why should the government be able to tell me what to view?" but I think these are weak arguments unless:
a) you feel the same way about viewing, downloading and distributing child pornography (i.e you think it is none of the government's business)
or
b) you can demonstrate that the harms caused by, say, chopping someone up with a chainsaw live on video, hacking someone's head off, or rampaging around a mosque and shooting men, women and children dead, are not to the same level as the exploitation of children.
Somehow, I don't think many people would really take (a) or (b), so I would be interested in what other arguments they have for arguing for their right to watch snuff films.
I also think the Nirvana Fallacy argument of "Well, you can't stop everyone watching it" also falls if you compare the ways in which avid seekers of child pornography will mask their identities online as well.