For some people, it’s the helplessness factor. Like if he did this to a 5 yr old, “the outrage” would be even larger. I personally think they came across disgusted at him murdering a person, but of course what started their obsession was the cats, which makes sense, as most people would never be exposed to a video taped real murder like they could the abuse of animals.
edit: in the no life comment someone made, it’s no different than if some folks took their time posting here and used those hours for detective work. And one chick said she fell into this while depressed, I was depressed earlier this year and know the more you can sink into something not about you, the more you avoid the terrible feelings. That said, they did this for two years