How do I cannibalize people?
You'll get the ability to eat any humanoid creature by completing a quest chain that results in acquiring the Ring of Namira.
In Markath, you'll meet Kleppr and Frabbi, who will tell you a rumor about the Hall of the Dead. This sets up a quest chain that eventually leads to meeting Namira, who will give you a choice as to whether or not you should consume your first dead body.
If you do, she'll grant you the ring, and when wearing it, you'll gain the cannibalism ability.