Lincoln was President during a turbulent time and he was on the cusp of a new era of thinking. To me many knew that slavery was wrong and that blacks were just as human as whites, however societal norms are hard to rebuke.
When we look at the Bible, we find the truth the inner need for justice we feel is given to us by God. We find that even when society has turned its back on a particular group, that the heart of many will hurt for the oppressed. This isn't an evolutionary survival tool, but something built into us by God. Lincoln and many others were conflicted, while they had been raised and told one thing, something inside them told them another. Judging them by today's standards leaves much room for error.