Aristotle has a simple answer to questions about the morality of rhetoric: he distinguishes the rhetorician and the sophist. What sets the sophist apart from the rhetorician is "not the faculty (dynamis) but the moral purpose (prohairesis)" (I1.1355M7; see de Soph Elen 1.165a30). Keep straight the difference between sophist and rhetorician and all moral problems will evaporate. He certainly doesn't think telling them apart needs great philosophical development or exquisite ethical judgment. Distinguishing them requires neither phronesis nor familiarity with the Rhetoric. He gives his distinction all the explanation he thinks it needs by saying:
In rhetoric, the person who acts in accordance with knowledge (kata ten epistemeri), and the one who acts in accordance with purpose (kata ten prohaireseiri), are both called rhetoricians; but in dialectic it is the purpose that makes the sophist, the dialectician being one whose arguments rest, not on moral purpose (ou kata ten prohairesin) but on the faculty (kata ten dynamin) (b19-22).
But his distinction between the rhetorician and the sophist seems too offhand for such weighty issues. We have to wonder why he thinks it adequate.
Issues of morality of rhetoric are worries first about the relation between its guiding and given ends, and then between the artful and and ethical guiding ends of praxis.
Aristotle's discussion of truth-telling, in the Ethics, helps explicate the distinction between rhetorician and sophist.
Garver, Eugene, "Deception in Aristotle's Rhetoric: How to Tell the Rhetorician from the Sophist, and Which One To Bet On" (1993). The Society for Ancient Greek Philosophy Newsletter. 231.