I think the concept that animal can be moral / immoral is already a philosophical subject. See the book "Can Animals Be Moral?" for reference.
And in order to say that these monkeys are evil, you must first prove that they can be moral.
If evil is absolute, then an animal (or an entire species) can be seen as evil by nature. Another species may be seen as good or neutral by nature even if the animal itself has no sense of right or wrong.
It is indeed very philosophical and also up to your beliefs and values.
I doubt it
"Law of the jungle" and all that