That is the only answer supported by the evidence.
If Lincoln was found to be fighting and killing to “free the slaves” he would have been guilty of the violent overthrow of the U.S. constitution - which enshrined slavery. And I don't think you want to go there.
I don't want to go there because there is nothing to support the claim.
That said, there is actually evidence that Lincoln did intend to violently overthrow the constitution.
That evidence being?
Lincoln's Gettysburg address.
Admittedly, that can be countered by any number of other Lincoln utterances over the years that implied the opposite.
Lincoln was an absolute master at making one audience believe he meant one thing, while making another audience believe he meant something different. He was every bit as good a retail politician as President Clinton, and perhaps better than President Obama. Very slick.
My best guess about “Lincoln fighting to free the slaves” - this was a scabbed-on justification for the war after the casualty rate could no longer be justified by the North's need to collect taxes on Southern wealth, some of which was based on slave labor. “Freeing the slaves” was a way to freight the killings with meaning.