It wasn’t all about slavery then or now. That is just a post-hoc rationalization peddled after the war. It was certainly partly about slavery.
Given that the War did happen, it was a tragedy that Lincoln didn’t survive. He would have been far better post-War than the Radical Republicans who followed.
Well, whenever you find yourself uncertain about the role of slavery in all this, just read Mississippi's so-called declaration of "immediate causes" of secession for a reminder:
"Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world."
Those words, written when they were by the secessionists themselves, can hardly be dismissed as a post-hoc rationalization. So, whenever you find yourself doubting the role of slavery, just go back and read again what the so-called secessionists themselves had to say from the beginning.
Despite the fact that we are now living in the 21st century, there are a few folks out there who maintain that they find themselves crippled and unable to function in any normal way today because their great, great, great, great grandparents were held as slaves in the 19th century. And, we even have a few folks out there who maintain that the choices that we make today in the 21st century about the size and scope of our central government were all fixed and determined by Lincoln, a 19th century politician. What an attitude - we're all cripples! And, it's hopeless. And, it's all Lincoln's fault!
As the "secessionists" argued in their declarations of "secession," the loss of slaves as property was a huge financial loss. It must have been awfully painful at the time.
But, it's long past time for all of us to move on now. Slavery is gone, gone for good - it's not coming back.