History have shown that humans will do anything to survive, no matter the cost for their beliefs, ethics or moral. If food supply ran out during winter and hunters fail to hunt any animals out in the wilderness and dying to predators, the settlers are forced to make tough decisions. When the first unfortunate human dies during a famine, in many historical accounts cannibalism been the last resort of nourishment for the people in a desperate act of survival.
For the game I believe cannibalism should be an event that can happen when there is no source of food left, and the first settlers begin to die from starvation. This will result in extreme decrease in happiness, and people are forced to eat the recently deceased. Granted if firewood is available, they will try to cook it to prevent diseases.
Cannibalism can also happen to be voluntarily, such as deceased raiders after a failed raid committed against your village, you can choose to butcher the corpses rather than having them rot in the middle of your roads or bury them. This will then result in a severe negative impact of the average happiness, and can result in potential trait changes. Some with certain traits (such as cannibal) will instead have a positive impact on happiness instead of a negative one.
TL;DR Use human corpses for food at the expense of massive happiness loss, and potentially having some settlers become cannibals by choice.