Lincoln wasn’t the hero he is typically made out to be.
He offered to keep slavery legal to keep the Union together.
We just have a lot of crappy history teachers.

Actually, his Republican party was opposed to slavery, based on “all men are created equal” and wanted at least to stop the expansion of slavery into the western territories. But Lincoln, although he came from a ‘free’ state, he was accepting slavery as part of reality. In fact, even before being inaugurated, when South Carolina was threatening to secede, Lincoln made a few speeches explaining that he was not in agreement with his party, and he was not opposed to slavery nor the expansion of slavery.
It was not till he felt the northern states, most of them ‘free’ states were becoming disheartened by the war, that he decided to “free the slaves” and make that the new cause for the war. This was mainly to make confederate troops want to return home to protect their families because of fear of their safety, and to encourage blacks in the south to rebel and disrupt the infrastructure in the south. Lincoln was careful to not free the slaves of the 4 states that remained with the union, he only freed slaves in states he had no jurisdiction to. (kind of like if Obama was to declare a minimum wage increase for workers in China, it really means nothing)