I'm not sure what Cotton meant by his comment, but when I read it, I thought of the John Adams miniseries I re-watched this past spring. Adams, Jefferson and Franklin had to take the anti-slavery clause out of the Declaration of Independence in order to get the Southern states to sign onto it:
Dr. Benjamin Franklin: We've no choice, John. The slavery clause has got to go.
John Adams: [stunned] Franklin, what are you saying?
Dr. Benjamin Franklin: It's a luxury we can't afford.
John Adams: [pause, then] 'Luxury?' A half million souls in chains... and Dr. Franklin calls it a 'luxury!' Maybe you should have walked out with the South!
Dr. Benjamin Franklin: [dangerous] You forget yourself sir. I founded the FIRST anti-slavery society on this continent.
John Adams: Oh, don't wave your credentials at me! Maybe it's time you had them renewed!
Dr. Benjamin Franklin: [angrily] The issue here is independence! Maybe you have forgotten that fact, but I have not! How DARE you jeopardize our cause, when we've come so far? These men, no matter how much we may disagree with them, are not ribbon clerks to be ordered about - they are proud, accomplished men, the cream of their colonies. And whether you like them or not, they and the people they represent will be part of this new nation that YOU hope to create. Now, either learn how to live with them, or pack up and go home!
I don't know if that's what Cotton had in mind when he called slavery a necessary evil, but if the portrayal in the miniseries is historical accurate, that is exactly how some of our Founding Fathers viewed it at the time:
Edward Rutledge: [In the final vote for Independence, Rutledge wants the slavery clause removed from the Declaration, or else he will vote against independence] Well, Mr. Adams?
John Adams: Well, Mr. Rutledge.
Edward Rutledge: [stands] Mr. Adams, you must believe that I *will* do what I promised to do.
John Adams: [stands and approaches him] What is it you want, Rutledge?
Edward Rutledge: Remove the offending passage from your Declaration.
John Adams: If we did that, we would be guilty of what we ourselves are rebelling against.
Edward Rutledge: Nevertheless... remove it, or South Carolina will bury, now and forever, your dream of independence.
Dr. Benjamin Franklin: John? I beg you consider what you're doing.
John Adams: Mark me, Franklin... if we give in on this issue, posterity will never forgive us.
Dr. Benjamin Franklin: That's probably true, but we won't hear a thing, we'll be long gone. Besides, what would posterity think we were? Demi-gods? We're men, no more no less, trying to get a nation started against greater odds than a more generous God would have allowed. First things first, John. Independence; America. If we don't secure that, what difference will the rest make?
John Adams: [long pause] Jefferson, say something.
Thomas Jefferson: What else is there to do?
John Adams: Well, man, you're the one that wrote it.
Thomas Jefferson: I *wrote* ALL of it, Mr. Adams.
[stands and goes to the Declaration, crosses out the clause]
John Adams: [snatches the paper from Jefferson and takes it to Rutledge] There you are, Rutlege, you have your slavery; little good may it do you, now VOTE, damn you!
Edward Rutledge: [takes the paper] Mr. President, the fair colony of South Carolina...
[looks at Adams]
Edward Rutledge: ... says yea.