who are these people, where did they come from

My guess would be Indonesia, or Southeast Asia, as that area has a large monkey population.

why are they doing this?

From the comments, and being from part of the world where monkeys aren't prevalent, I was thinking people from certain parts of the world may see monkeys as vermin. Still very weird that people actively seek out these types of videos though, but I've seen videos of mice/rat catching techniques that get quite popular as well, which some would think of as cruel (culturally we are much more likely to look past it, as mice/rats are deemed pests).

It may be a case of culture shock, that in some places animals we view as fascinating aren't held in the same regard. Consider the treatment of dogs in some Asian countries, which would shock and outrage many people. Though, any instances of abuse should be taken seriously by youtube, and you should report it.

Is there a pedophilic/sexual background to it?

Searching what you've provided, it doesn't look to be the case.