How Hannibal Lecter Become a Cannibal?

In a haze of rage and grief, Lecter stalked and killed the butcher with one of Lady Murasaki's samurai swords, eventually cutting out his cheeks and eating them. This was Hannibal's first kill, and his first act of cannibalism by choice.

Why did Hannibal eat his sister?

It was later explained in Hannibal Rising when he is about to take revenge on the last soldier he hunted down who tells him he was just as eager to eat her as the rest of them were… Giving him his acquired taste for human meat that he doesn't want to accept was because he ate his sister to avoid death.

What did Hannibal Lecter suffer from?

The victim of childhood trauma involving the killing of his family and the cannibalization of his baby sister, Lecter suffers from posttraumatic stress disorder.