Well, the creation of the video involved a horrific crime, but now that it's been created, there isn't necessarily additional harm in sharing it. Admittedly, there are potential harms: the more popular the video becomes online, the more it may inspire future terrorists, both to commit terrorism in the first place and to livestream it. On the other hand, though, it's always been true that regular news coverage of terrorism and the resulting wide public attention can inspire future terrorism, yet nobody has proposed clamping down on that. At most, some propose to avoid saying the killer's name in order to make the infamy less personal, but that's a half-measure at best. In particular, I bet the livestream made more impact on the public through the news using it as an additional gruesome detail to talk about than through people actually watching it. (Counterpoint: If platforms hadn't censored it as heavily, more people would have watched it, and if they had more effectively blocked reuploads, fewer.)

There's also the factor that the killer knew his video would be shared and that that probably formed part of his motivation for committing the crime in the first place; regardless of its effect on future crimes, sharing his video sort of retroactively justifies his decision to commit this one, and indeed may make him more satisfied while in custody. But I don't particularly care about the killer's present level of satisfaction, and the presence or absence of retroactive justification will not affect the past.

Then on the opposite side, what's the benefit of sharing it? Not much, to be sure. But some people have morbid curiosity. Personally, I vaguely considered watching it just to test how I'd react to a real-life version of the kind of violence I've seen so many times in fiction: whether I'd be able to maintain a similar emotional detachment (knowing that the horror was real, but also that my watching it or not wouldn't change what happened), or whether I'd get overwhelmed have to shut it off. I said I vaguely considered it: I quickly decided that such an experiment would be really ill-advised, given that even minor psychological harm to myself would vastly outweigh the trivial benefit of satisfying my curiosity.

See also Andy Carvin's account of actually being diagnosed with PTSD after years of watching horrific videos in his role as a reporter:

https://medium.com/dfrlab/dont-watch-the-new-zealand-mosque-...

...And yet part of me preferred to be able to make that choice for myself, and I wouldn't judge anyone for making the opposite decision.

As for child pornography, the situation does have some parallels – creating the photo or video is generally (see later in paragraph) a horrific crime, sharing it not quite as much. One large difference is that if the victim is still alive (whether or nor they're still a child), additional people watching further victimizes them. Partly because it may end up affecting their lives in the future, if any of those people go on to interact with them, but partly because they just have an inherent privacy right not to have people getting titillated at them, regardless of whether it directly affects them. In that respect it's similar to revenge porn, where the original creation of the photos/videos may have been done with the subject's knowledge and consent, in which case it's not morally wrong at all, but weaponizing them by posting them publicly is wrong (and a crime in an increasing number of jurisdictions). Same may apply to photos/videos of children which were created [supposedly] without sexual intent but later shared with it (the reason I said "generally" before).

If the victim is dead, it's a bit more morally thorny, but still quite different: unlike with terrorism, you don't have the news as a legitimate 'side channel', so the connection is less attenuated between sharing child pornography and others being inspired to make more of it.