Why is Lincoln “great”? Because he was incapable of averting a war that killed 500,000 Americans? Because he ended self-government in a whole region of the United States?
Any like Kennedy, he never lived long enough to show just how "great" he could have been :)
Lincoln saved the Nation from disintegration and helped put a stop to slavery. The words, Mercy, dignity and honor for his oppenents come to mind.
It is interesting to think of what the War would have been like if the all but a few of the Regular Army Officer Corps had not been Democrat, and opposed to latching on to the Confederate Army until it surrendered or was wiped out.
I suspect Lincoln is great because, unlike 99% of our "leaders", then and now, he was not in politics for the money,