Are you talking about Lincoln or Congress, or both?

In what way would Lincoln have been justified in freeing slaves (which was, in fact, confiscating property) of loyal Unionists during the war? I agree that as commander-in-chief, he had the power to take extreme actions for military reasons. But, what military reason, or wartime necessity, was there to justify the forcible freeing all slaves across the nation?

Your comment that "the fact it took [Congress] from 1861 until 1866 to end slavery IS inexcusable," is a personal and 21st century judgment of 19th century Americans...and one that arguably ignores the political realities of the 37th and 38th Congress and tenuous border state balance during the war. In 1861, concerned legislators in both houses passed version of the Crittenden-Johnson Resolution (formulated by John J. Crittenden of Kentucky and Andrew Johnson of Tennessee) which declared that the federal war effort would not overthrow or interfere with slavery. Though many Republicans eventually rejected this resolution, similar concern for Kentucky, Missouri, Maryland, and Delaware support bothered moderate Republicans during the increasing move against the institution. Many people point out how unpopular Lincoln's Emancipation Proclamation was at its announcement and even after its implementation across the North (let alone the border states), particularly among Democrats. To say that Congress should have abolished slavery sometime between 1861-1863 completely disregards the sentiments of most Americans during that time. One might as well say that Congress "should" have abolished slavery in 1800 or even 1787.

Congress, as a body, always had the authority to pass an amendment abolishing slavery. What is important is when people in Congress were willing to use that authority for that end.

As I said before, in 1860 only a small population of radical abolitionists supported a sweeping national ban against slavery. Following events so dramatically changed American perceptions that by 1865---in only 5 years---the federal government (and most northerners) was pushing an emancipation amendment. This transition was not "inexcusably" slow, but remarkably swift.