I think this is really a different situation. The NZ video was made by the perpetrator as a vehicle to disseminate his views. 9/11 videos were made by bystand

I think this is really a different situation. The NZ video was made by the perpetrator as a vehicle to disseminate his views. 9/11 videos were made by bystanders as they watched their city being attacked. A better comparison would be videos produced by terrorist organizations which use 9/11 footage to promote their own views: that would be something I could see being argued for a ban.