Ok, my guess is this is definitely some sort of parasite from the monkey. The woman from the diary in the last chapter mentioned feeding the monkey “mouth to mouth”, so it probably spreads by fluid (Or by eating the monkey, I guess. i guess they didn’t cook it enough?).
They mentioned the “outcast“ monkeys (hermits?) were abnormally hungry and very promiscuous, same as her boyfriend was in the first chapter. Guessing the parasite makes you eat a lot (to feed itself) and be promiscuous (to spread).
my guess about the whole angel connection: this chapter hints that angels are based on birds of prey. Maybe the parasite’s adult host is actually birds? like that breed of snail parasite that makes the snail climb higher up and wait for a bird to come eat it, maybe the “angels” are how the victims are perceiving the “climb a tree and wait for a hawk” signal. Though what bird would eat a monkey?
my guess about the “they seek out their fears” thing is that the parasite isn’t properly adapted to humans - we’re close enough to monkeys that it can survive but not close enough that it can actually control them, so the “go and wait for a predatory bird and don’t try to escape” becomes “go skydiving without a parachute“ since our fears aren’t usually related to predatory animals.
Or I’m on the totally wrong track and the plot will reveal the explorers got cursed by the god of monkeys as punishment.
edit: now that I think of it though, my theory doesn’t explain why the victims say they literally hear angels chirping. It would be weird for a parasitic organism to make noises.