Commodus and his father
I love the scene when Commodus is informed by his father that he won't succeed him as emperor. All his emotions come bursting out him. Fear, resentment, insecurity and sadness. He feels that his father never loved him nor did he appreciate him. Then came the anger which results in him taking drastic action. Great acting from Joaquin Phoenix.
This is similar to that scene between Michael's and Fredo in The Godfather Part 2. Fredo can no longer hold it in and tells Michael how he has always felt. Being passed over fuels resentment that can fester for years.