"I am naturally anti-slavery. If slavery is not wrong, nothing is wrong. I can not remember when I did not so think, and feel. And yet I have never understood that the Presidency conferred upon me an unrestricted right to act officially upon this judgment and feeling."
He was very much against the expansion of slavery. This is clear in his writings and his voting record. I'm not saying that he was an outright racist, but take him in historical context. He resisted his party's call for emancipation for over two years into the Civil War, and he only issued the Proclamation once the North was all but sure to lose. And even then he only abolished it in states that were rebelling against the Union. He could never have abolished slavery outright given that there were six slave holding states that were fighting on his side that would have seceded immediately thereafter.
Lincoln repeatedly asserted that the President possessed no legal power to rewrite the United States Constitution. The only way he could justify emancipating the slaves was in his capacity as Commander in Chief of the armed forces, which meant that he had to wait until it became a military necessity.
He had already exercised his power as Commander in Chief by sending troops into combat. He didn't have to wait for it to become militarily necessarily -- the Constitution gives the President wide berth in this regard. It was reigned in with the War Powers Act in 1973. I'm not sure he was much of a strict Constitutional kinda guy either, as the Founding Fathers made allowances for states to secede, or reject the Union in whole. New York made sure they could back out of the federal government at any time before ratifying.