
How selfish, stupid, ungrateful, etc. can you get?
Expression of Disbelief
Used to express shock or disbelief at someone's extremely bad behavior. 
Rhetorical Question
It's a rhetorical question, not expecting a real answer, but to emphasize the point. 
Stress and Intonation
Emphasis is often on 'how' and the adjective, reflecting disbelief or criticism. 