I think it's about detachment. I consume a lot of dissection, postmortem, surgical, etc content.

As long as I can surpress thinking it was a real person, I can watch it without any problem. After many videos, it becomes automatic. Desensitisation is a real problem with watching things from a screen.

But key factor is control, I know what I am going to watch and I can control whether if I want to. Those people on moderation team may not have such situation.

OP is grossly misunderstanding about the long term cost, though. Short term reactions differ from person to person. Some people can quickly associate real things from a video while others don't. It's akin how people can become too mean and insult others online without feeling bad about it.

But things still float around in your head even if you didn't have an immediate reaction. You would remember it some time later or have dreams about it. It will be horrible.

Meh, western entertainment is already full of violence, torture, and murder. People get pretty good stomaching that, and I don't think it's very different if a video is real. As long as it's all happening away from you, people you don't know, events that don't affect you, it might as well be fiction.