Everyone that tries to make it about slavery is just a liar trying to lie their way out of the bloodshed they committed over a fake, made up "concern" for slaves.
Having allegedly gone to war to end slavery, the North could not make slavery illegal in the Union states where it was legal until after the 13th Amendment. Had the North won the war in a week or a month, it would have had no way to legally prohibit slavery in any of the slave states absent a constitutional amendment.
Emancipation Proclamation:
[excerpt]
Now, therefore I, Abraham Lincoln, President of the United States, by virtue of the power in me vested as Commander-in-Chief, of the Army and Navy of the United States in time of actual armed rebellion against the authority and government of the United States, and as a fit and necessary war measure for suppressing said rebellion, do, on this first day of January, in the year of our Lord one thousand eight hundred and sixty-three, and in accordance with my purpose so to do publicly proclaimed for the full period of one hundred days, from the day first above mentioned, order and designate as the States and parts of States wherein the people thereof respectively, are this day in rebellion against the United States, the following, to wit:
Lincoln expressed his war power as Commander-in-Chief of the Armed Forces. The Proclamation alleged a fit and necessary war measure to suppressing rebellion. The same Lincoln had rejected this as a fit and necessary measure at the outbreak of the war.
There was absolutely no legal bar to abolishing slavery in the District of Columbia. At any time, all it took was for Congress to act. It took a year for the Union to abolish the slave trade in D.C. The public sale of slaves down the street from the White House was going to be embarrassing if the basis of the war changed from saving the Union to freeing the slaves.
In point of fact, the Union purportedly fighting a war to end slavery, did not legally abolish slavery in D.C. until April 16, 1862 (12 Stat. 376), and did not legally abolish slavery in the Union states until December 1865. Despite the 13th Amendment, New Jersey stubbornly resisted abolition until January 1866.