The only ones who tend to speculate on the true nature of Lincoln's character in order to cast him in a bad light are those who are more likely to wear white hoods and burn crosses. The amount of first hand documentation we have on the Civil War, and on Lincoln himself, is so extensive it's mind boggling. There have been over 400 books written about Lincoln- more than on any other historical figure in history. If any man deserves the high praise history has bestowed upon him, it's Lincoln.
Without disputing that Lincoln is one of those rare figures who deserves to be as famous as his reputation suggests, that abundance of documentation doesn't mean that there isn't room for doubt about who he was or what he did.
For an utterly trivial example, consider: we do not know exactly what he said in his address at Gettysburg. The drafts of his speech are not perfectly consistent, and wouldn't bind him rigorously to the exact words he used, and newspaper accounts vary, in some cases enormously. What the crowd reaction
to the speech was is impossible to determine given the abundance
of inconsistent documentation available.
If we can't know with reasonable certainty the proceedings of three minutes, witnessed by thousands of people and reported on widely in the press at the time, and remembered by its witnesses for decades after, there's room for reasonable people to question the great themes of his life.