In Google's case, maybe not.
When I worked with the police, yes. You needed every scrap of horrible information and context. Usually because it's much worse than you think initially. (Background voices tell you someone was forced to watch, etc).
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There's a difference between a quick slaughter for food and torturing something to death for fun.
I wonder if the job truly requires these contractors to watch these videos in their entirety. If so - why? Regarding the above, surely you'd only need to watch up until the moment the man broke the dog's back to determine that the video was not "advertiser-friendly".