There's a cogent argument which would claim that proliferation of the Christchurch shooter's footage would serve as a specific causal factor for future mass shootings, especially because part of the motivation of that shooting was exactly the proliferation of that manifesto. In that sense it's almost a memetic threat because any proliferation of the video locks in the reward structure for future copycats. In terms of discourse about what should/shouldn't be generally exposed to people with only a passing interest (external to any discussion about how that moral imperative aught to be enforced), the Christchurch footage would be pretty close to the top of my blacklist.

Moreover - I'm pretty sure anyone with a modicum of net-literacy could find that video with relative ease if they really wanted to. It's clearly not a binary of proliferation and censorship, but a complex gradient where different mediums of curation have different thresholds for that kind of exposure.