Fiction books that explore the primal, animalistic urges/instincts of the human race? (Think Lord of the Flies)

I’m looking for novels that talk about the “lizard brain” basically.

I’m looking for books akin to Lord of the Flies that explore the animalistic darkness within every human. It is my understanding that the real life stranding that inspired Lord of the Flies went a bit differently in real life, but I still find it an interesting topic.

As pessimistic as possible, please.