I can't speak for the other signers, but Thomas Jefferson, the author of the Declaration, was an idealist who didn't always live up to his own ideals, including but not limited to slaveholding.

Jefferson, 1789: I am not a Federalist, because I never submitted the whole system of my opinions to the creed of any party of men whatever in religion, in philosophy, in politics, or in any thing else where I was capable of thinking for myself. Such an addiction is the last degradation of a free and moral agent. If I could not go to heaven but with a party, I would not go there at all.

Also Jefferson, 1792: (founds political party to oppose the Federalists)

Machiavelli, ca. 1513: for many have pictured republics and principalities which in fact have never been known or seen, because how one lives is so far distant from how one ought to live, that he who neglects what is done for what ought to be done, sooner effects his ruin than his preservation; for a man who wishes to act entirely up to his professions of virtue soon meets with what destroys him among so much that is evil.