At the heart of the arguments here are moral and emotional ideas of guilt and innocence, rather than more impersonal or objective concepts of causation or development. Also, there's a desire for clear answers and unambiguous characterizations. What was Woodrow Wilson's line on WWI? What was Churchill's attitude towards Stalin or Hitler? Or Reagan's approach to taxation? The answer is that these things changed over time. To be sure, there were constant convictions in the minds of such men, but practical policies changed as circumstances and opportunities changed.
So it was with Lincoln's attitude towards slavery. What was possible and desireable at the time changed as circumstances changed. But in contrast to many other politicians of the day, Lincoln did have a bedrock conviction that slavery was wrong, though practical accomodations would have to be made to circumstances and changing priorities.
We demand that everything be subjected to moral convictions that we have already come to agree on. But is that the case with contentious issues in our own day? Were there is no consensus, policy can't take on contentious questions head on. It has to procede by zigzags and half-measures, a step backwards for two forwards.
The controversy also gets complicated, because slavery was the issue in the 19th century, and people today are talking more about racial equality and integration, which were very radical ideas at the time. It was too much to ask for any serious candidate to office to support racial equality.
Go in peace, my friend, lest you too be drug into the dismal swamp of defending the obvious to the jeers of the oblivious.