The motive in the majority of cases of a small animal or child being abused and tortured to such a disgusting level is almost always power. The idea that you have this small, vulnerable creature dependent on you for survival/food/love/whatever gives these people a sadistic rush. Notice that most zoosadists always prey on young dogs and cats, rodents, birds... None of these sick fucks are going to try torturing one of those big-ass police K9 type dogs, or something like a deer or horse that can fight back and actually fuck them up. At least, not without ensuring they've removed the ability for that creature to resist whether by drugging them, restraining them, etc. As for why they experience this urge, who knows. It could maybe be a result of trauma or past abuse, but getting to the level of Kero or the monkeyfuckers tends to require some serious pre-existing issues. To put it another way, young kids who grow up to be serial killers torture the neighborhood cat because weak animals like that are the only feasible targets to satisfy that urge for a power trip through violence and sadism at their age. What we see in adults who engage in similar behavior, whether with small animals or with children/other people/etc. is just an extension of that, but now that young future serial killer is an adult with the ability to exert dominance over more than just the local wildlife. It's sickening.
This is likely more relevant for the producers actually making the shit, of course. As for the consumers? That's a whole separate can of worms. The same desire for power is probably there, but I assume there's more detachment. These fat white suburban housewives that get off to monkey gore on the down low likely experience the same rush at the idea of exerting their will over a small creature via torture and abuse, but they don't have the... balls, if you will, to really do it themselves. So, they pay some rice farmer living in Southeast Asia to do it for them and send them the results. Logically, we obviously know it's no different than doing it themselves, but that level of distinction likely makes it easier to justify in their own heads. "I'm just watching videos, right? I'm not actually torturing the monkeys myself! I'm still a good person!" Also just a side note, I have a feeling monkeys are such a popular candidate for this shit not only for the reasons this thread has already gone over (similarity to humans making monkey abuse closer to actual snuff/CSAM in some ways) but also because... Well, look at the people consuming this shit. The majority are people who own dogs/cats and have probably never seen a monkey in their lives outside of the zoo. It's likely easier for them to detach from the experience emotionally when they see this strange foreign creature being tormented instead of, I don't know, a puppy with the same fur pattern as the one they claim to love at home.
Note that this is all just speculation, by the way. These are just the observations I've picked up after reading so much about these fucking monsters. At the end of the day, to truly understand their mindset would require us to be just like them, and I'd rather eat bullets than willingly consume the absolute evil shit these "people" get off to.