Scarcity of food
When food becomes very scare some people turn to cannibalism in order to survive. This happened on a wide scale in the siege of Leningrad.
To absorb their powers
Some people become cannibals for cultic reasons. In Papua New Guinea a group of people were arrested after eating some humans they killed. Those cannibals believed that by eating those evil people they can absorb their supernatural powers.
To stop sorcerers
Some cults believe that by killing sorcerers, or the one’s they think are evil, and eating them they can prevent them from using their Evil powers again.
Some people are Cannibals because it’s a normal part of their culture to eat other humans. Some tribes for example celebrate victory by eating the bodies of their enemies.
To absorb their characteristics
Some tribes believe that by eating the bodies of some dead people they can actually absorb some of their characteristics.
To guide their souls to living bodies
Some tribes eat the bodies of their dead ones in order to guide their souls to living bodies. This can be a part of the grieving process.
According to Wikipedia. some mentally ill people can get sexual satisfaction from eating the flesh of the dead victims they killed. Wikipedia gave the examples of Albert Fish, Issei Sagawa and Jeffrey Dahmer. (See Why Sex is Addictive?)
A form of medicine
In the 16th century, eating the bodies of mummies was believed to be a cure from some diseases. That fad ended when they discovered that some of those mummies were the bodies of recently killed slaves.
To seek vengeance
In some cases, cannibalism can be an act of vengeance. Some people ate their enemies after killing them in order to humiliate them.
To terrify opponents
Cannibalism can be a method used to terrify opponents. For example, an army or a tribe can use cannibalism to scare the other group they are fighting.
Twisted way to feel powerful
Some of the cannibals caught described feeling extremely powerful as they removed human flesh. This gives some mentally ill people a feeling of satisfaction and superiority. (See Why do people strive for superiority?)
To end loneliness
Some cannibals feel extremely lonely. As a result of a twisted belief they think that by eating someone they can keep them around all the day.
To feel in control
Some cannibals eat others in order to get an illusory sense of control over their own lives.
Why do cannibals eat humans?
Why did cannibals eat other people? Why do cannibals love eating people?