Um, because it was monkeys that did it, not humans? And humans =/= to monkeys? It's surprising because (a) we generally consider revenge to be a human emotion,

Um, because it was monkeys that did it, not humans? And humans =/= to monkeys? It's surprising because (a) we generally consider revenge to be a human emotion, so seeing it in animals is a bit surprising, (b) seeing a coordinated attack between two species that are kind of adjacent on the food chain isn't super common. Killing in the animal world is primarily done for hunting. The monkeys aren't eating the dogs, they're just killing them.