Consider this excerpt from Lincoln's First Inaugural Address (emphasis mine):
Lincoln plainly stated that the ONLY reason he would invade the South would be to enforce the tariff laws. Thus, unless one is an extremely passionate proponent of protectionism, it makes no sense to allow the destruction of one's hearth and home rather than attempt to repel Lincoln's invading forces.
Except it doesn't even say that.
It clearly says that Lincoln would also invade if the federal government's property (such as, say, Fort Sumter) was tampered with.
Quite apart from that, the first inaugural address had an important role as propaganda in convincing the border states to remain in the union, so shouldn't be taken as summing up all of Lincoln's war aims.
Though, if you do want to selectively quote from Lincoln's speeches, you could read this, from his first inaugural:
Or, maybe this one from his second inaugural:
The question of whether slavery was the main cause of the war is not one serious historians debate.