Someone here has once presented a pretty plausible theory for their fixation on baby monkeys, in particular the longtail and pigtail baby macaques who seem to have taken the brunt of the abuse on the videos (from my own observations in their groups, they detest torture of other primates and animals. Someone once posted dog torture and he got b& pretty fast with a lot of outrage). It's apparently all to do with the baby macaque's similarity to human babies, which kinda ties in with the whole "uncanny valley" theory. Many people, especially women (who make up a CONSIDERABLE portion of the monkey torture community), find human babies annoying, needy, loud etc. The frustration of raising a baby at home is pretty intense at times, as all parents can personally confirm. Naturally, expressing anger towards human babies is a huge red flag that will get them hated by everyone, so they found an outlet in baby monkeys as effective proxies. They feel like they can do to baby monkeys what they can never do to a child out of frustration, and get away with it.They couldn't be more obvious about that motivation if they tried. I don't pay much attention to zoosadism because it is hard to stomach, but if you look the monkey stuff the people paying for it will want them in diapers and stuff like that. They'll have them get tortured as punishment for "bad behavior" and things like that. The way the sadists talk about the torture anthropomorphizes the animal getting tortured. It's generally women who do from my limited observation this and if they were dudes they'd probably be the fucked up kid who you heard killed animals and stuff out in the woods, so you steered well clear of them growing up.
Also, another point to remember is that there are many monkey fuckers out there, and they all have different motivations for using these baby macaques as proxies for some desire they have. Some are frustrated mothers (see above), some are pedos (diaper play and penetration), some are edgelords (looking for the most taboo things to do online for clout) and gore fiends (just general fucked up mutilation, cartel vids, war video addicts etc), some are loners who want to belong to a community and found one in a cesspit of the internet, those are the dolts you see arrested over FB posts. Ofcourse there are also the psychos who kill birds and cats in their local areas. A happy, cheery community XD