Aposiopesis is a rhetorical device which involves a breaking-off of speech, frequently because of rising emotion. The speaker leaves the conclusion of the statement to the imagination of the audience. Aposiopesis is typically denoted with an ellipsis or a dash.
Examples of Aposiopesis
"If you weren't family...!"
"Why, I oughtta...!"
"One of these days...!"
"Or else...!"