To the contrary, you can examine collections of his complete written works, speeches, letters, and more online (and in a Gutenberg edition) where the subject of slavery is brought up many many many times before his election.
It was THE dominant controversy of the age.
on the other hand, the slave states decided they would seceed if he was elected . . . .
And the decision was forced on him by the sheer magnitude of the war.
see this discussion
of course, is it a greater crime to free the slaves, or let them stay enslaved in order to avoid a war.
There is a long tradition in theology and elsewhere about the concept of a "Just war"
see
https://en.wikipedia.org/wiki/Just_war_theory
Is fighting to free a nation of slaves (millions of people) suitable justification for war?
If you are fighting a war, is the freeing of a nation of slaves (millions of people) a justifiable tactic?
I think that's besides the point. Slavery would have died out on it's own. Lincoln started the war under the guise of keeping the Union intact, it was not until the war was already started he interjected the slavery issue.