What figure of speech or rhetorical device is exemplified by Ceasar's famous "Et tu, Brute?-Then fall Ceasar!"William Shakespeare's Julius Caesar

In Act III, Scene 1 of Shakespeare's Julius
Caesar as the conspirators approach and Caesar asks why Brutus kneels
uselessly, Casca exclaims, "Speak hands for me!" and he and the others stab Caesar. 
Caesar sees his friend Brutus and asks the rhetorical question, "Et tu
Brute?" which is Latin for "And, you,
Brutus?"

A rhetorical question differs
from another type of question used in rhetoric, hypophora, in that
it is asked without the expectation of a response.  Caesar, completely surprised by the
conspirators, is even more amazed to see Brutus as part of them.  The reader will
remember that Caesar has mentioned to Marc Antony in Act I that he is wary
of the appearance of Cassius who has a "lean and hungry look."  However, Caesar has
never entertained the idea that Brutus may wish to assassinate him; instead, Caesar
believes Brutus an ally and friend.  His rhetorical question, therefore, expresses this
surprise and disbelief, as well as it expresses a disappointment in the noble Brutus. 
It is, indeed, more of an emotional expression that a question per
se.