I actually take some issue with that. I think publicizing violence can be a cause of violence (the same critique could be applied to 24/7 media coverage of a shooting tragedy, which is possibly just what the shooter wanted). That's really not a point worth having an argument about, I just thought I'd throw it out.

I think what that sentence actually means is "it's a threat to the edifice of safety we've created and potentially to the usability of our service", i.e. "People want to pretend this kind of stuff doesn't exist and certainly don't want to see it when they are watching cat videos." Honestly, I somewhat agree. If I thought I would stumble across a beheading video when watching YouTube, I'd probably skip it entirely.