If there is a complex enough set of moral truths, it would require a comparatively complex enough set of thoughts to "get at" these truths. However, it is possible to know what is good and not do it (acedia) as well as to know what is wrong and do it (akrasia), so there is a mismatch between "being intelligent enough to know complex moral truths" and "being virtuous enough to abide by those truths."
Now on the other hand, too complex a code of ethics and the code might not be "reliable" enough to act on. So we might not expect a plausible code of moral truth to require spectacular intellect to arrive at and work with. And there is an abstract similarity between truth and goodness such that people who "act contrary to the truth" (a phrase in Kant's labyrinths) might very well seem to therefore "believe contrary to the truth," too.
So the mainstream approach to solutions to this question takes the difference between all-things-considered judgments and all-out judgments as the point of departure: let us say something like "proof enough for knowledge is consistent with akrasia, but absolute proof is not." I.e. if you "use your intellect" to completely prove a point of ethics (supposing you ever could!), you can tie your intelligence to your subsequent lack of (in context) unethical behavior. But note that, to preserve free will (as choice), even absolute moral intellection will not compel you to the good alone, i.e. will not solve for acedia. Only the thing is, whereas before you could choose from "good, neutral, wrong," now you can only choose "good or neutral."