That's probably the scariest thing about disturbing violence. Ceasing to be disturbing is the bigger issue than being disturbing IMO.

But I'm weird and think that it's far better to let children see sex than even regular hollywood violence, and that things like US military recruitment ads and ISIS recruitment videos should be treated identically (restricted behind a warning that they're not for kids). The normalization of violence is a terrible thing.

Sure. As an adult you can still continue seeing it as wrong (if you did before). It will just not give you chills.

Trouble with children is that they may not yet have the moral compass for lack of a better phrase. I would find it important to talk to the child and ask him what he's thinking about the videos, and make sure he understands that the behavior in the videos is horrible, ends the life of another person, what that means, etc. I'd also make sure the child would not get trapped in some cycle of feeling guilty for simply watching some videos and having/not having whatever reactions to it.