What is Candide's argument against optimism in chapter 16?

In chapter 16 of Candide, Candide and his companion Cacambo come across the Oreillons in South America. They observe two monkeys chasing two girls, and Candide feels the girls are in danger, so he shoots and kills the monkeys. Much to his surprise, the girls cry, mourning the loss of their "lovers." It turns out that the chasing was just a playful, romantic game. As a result of his actions, Candide is sentenced to death by the Oreillons, partly because he is dressed in Jesuit garb (he steals Cunegonde's brother's clothes after killing him in a previous chapter). The Oreillons start to celebrate that they are having "Jesuit for dinner," which makes Candide reflect on Pangloss's optimism, the philosophy Candide has been taught his whole life.
Candide's response to being sentenced to death is to say,

We shall certainly be roasted or boiled . . . What would Professor Pangloss say if he had seen how unsophisticated nature behaves? No doubt it is all for the best, but I must say it is very cruel to have lost Lady Cunegonde and to be skewered by the Oreillons.

Candide's response is a good example of understatement, as he is fairly calm and collected for someone who's just heard he's to be cooked and eaten. He wonders what Pangloss would think of the Oreillons' behavior, since what Candide observes clearly strays from Pangloss's rosy view of the world. Candide pays lip service to optimism, saying, "No doubt" Pangloss is still correct, but he also recognizes that the events that have befallen him are nothing to be optimistic or cheerful about. Some small doubt has begun to creep in, and we see by the end of the novel that Candide has essentially put Pangloss's teachings aside and formed a more realistic view of how the world operates.