Child pornography is an industry; people make child porn specifically for the purpose of providing it to people who want to see child porn. They make money off it. Banning the possession or distribution of that kind of material is part of fighting the industry that creates it, and therefore is part of preventing the kids from being abused in the first place. Videos of killings generally don't fit the same model. Al-Qaeda's beheading videos certainly don't, nor do those hammer-murder videos.

IF someone was videotaping murders for the purpose of selling the video to people who want to see it, then I'd be okay with banning that, because banning that sort of video would make it less likely that it'd be produced at all. But snuff videos of that kind are really an urban legend.