Lincoln was not a slave owner, but he was a politician.. The whole issue of slavery really started in the 1850's with England and France importing cotton from the southern states of America. This demand for cotton, required more slaves to pick it. The invention of the Cotton Gin, required even more slaves. Plantation owners, and their politicians were intent on expanding slavery into the newly formed states west of the Mississippi. Anti slavery foes were against any new states allowing the owning of slaves. With the election of Lincoln, South Carolina became the first state to succeed. By the swearing in ceremony of the newly elected President Lincoln, a total of seven had succeeded and formed the Confederate States of America. So, because Lincoln did not want newly formed states in the west, to allow the expansion of slavery. The American Civil War began with South Carolina firing cannon shells into Fort Sumpter.
Lincoln never owned slaves and wanted to end slavery. He was one of the early Republicans, a party formed to end slavery and polygamy. During the Civil War he was more interested in preserving the country than ending slavery, so he waited on the Emancipation Proclamation until he thought he could get away with it. He only applied it to the rebel states, since they were or would be under martial law, giving a president (as commander in chief) broad powers to rule. The constitution didn't give him the power to end slavery in US states (states that hadn't seceded). Ending slavery everywhere in the US had to wait for the 13th amendment to the US Constitution.