The line, "I ate his liver with some fava beans and a nice Chianti," is a famous quote from the movie, but it's not a lecture. It's a chilling description of his actions.
The character of Hannibal Lecter is a complex and disturbing figure, and his motivations for cannibalism are never fully explained. However, it's clear that his actions are rooted in a deep psychological disturbance and a desire for power and control.