This is an "either/or" question that cannot be answered so simply.
Anti-slavery was a big part of the platform he was elected under, but it was not the whole of the man. He did not ask for the war, but he had the sworn duty, as President, to preserve the Nation. To that end, he bent all his efforts, and with the hindsight of today, I thank the Lord it was preserved. The evils of the 20th century would have crushed a divided America.
I admire him for doing his best to serve his Nation in the job he was elected to do. A lesser man would have broken.