Firstly, the initial war goal was to preserve the Union, not end slavery. The war goal changed in 1863 when Lincoln gave his Gettysburg Address, stating "Four score and seven years ago our fathers brought forth, upon this continent, a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived, and so dedicated, can long endure." It was unpopular because the North at the beginning of the war was suffering defeat after defeat. You will see this same situation for the South near the end of the war, when Sherman took the fight to the people during his "March to the Sea." In fact, the CSA introduced conscription a year before the Union did. After Gettysburg and the Fall of Vicksburg, Union morale was renewed.
Secondly, I don't understand where all this "Lincoln was a tyrant for not letting the South go," because it is the duty for every president, even if he/she is against the idea, to keep the country in one. It is an oath they all must take.