It was more than just his "goal" to preserve the Union; it was his duty.
I think Lincoln would have far fewer critics had he taken his oath of office to heart and applied himself to protecting and defending the Constitution. He had little use for that document and was much more interested in exercising powers not granted him by the Constitution.
Poor Lincoln. He married a madwoman, carried on an affair with a man for most of his adult life, and scoffed relentlessly at religion until the day he got what he deserved. To top it all off, he was the hero of Marx, Engels and the other early communists, and is venerated to this day by that same sort, who are the political descendants of the people in his own party who very likely plotted his death. Imagine his shock when he died and learned that the God he had mocked was real.
If he had it to do over, would he do as he did?