To be precise, while slavery was immensely profitable, continuing to fill the continent with oppressed people was a tinderbox waiting to ignite, and settlers on both sides were quite self-aware about that. It's hard to choose just one quote, but here's one from Settlers chapter 4 to explain this:

We can only understand the deep passions of the slavery dispute, the flaring gunfights in Missouri and "Bloody Kansas" between pro-slavery and anti-slavery settlers, and lastly the grinding, monumental Civil War of 1861-1865, as the final play of this greatest contradiction in the settler ranks. It was not freedom for Afrikans that motivated them. No, the reverse. It was their own futures, their own fortunes. Gov. Morton of Ohio called on his fellows to realize their true interests: "We are all personally interested in this question, not indirectly and remotely as in a mere political abstraction - but directly, pecuniarily, and selfishly. If we do not exclude slavery from the Territories, it will exclude us."

To millions of Euro-Amerikans in the North, the slave system had to be halted because it filled the land with masses of Afrikans instead of masses of settlers. To be precise: In the 19th Century a consensus emerged among the majority of Euro-Amerikans that just as the Indian nations before them, the dangerous Afrikan colony had to be at first contained and then totally eliminated, so that the land could be filled by the loyal settler citizens of the Empire.

I'd argue his opposition to slavery was out of morality but, his liberal obsession with protecting property rights made him never advocate for uncompensated abolition until he had near total power to do so, and then he still tried at first to pay the slave owners for emancipation (which they refused). He was always at least held antislavery positions for his entire political career, and for a long time held the same political positions on emancipation as his political hero Henry Clay.

If you want to criticize Lincoln's emancipation politics there, you can especially criticize his long-standing belief that America could not be a multiracial society, and that freedmen should all be sent to Liberia, something he also adopted from Clay. He pursued and promoted this colonization plan well into the later parts of the war. And while he became more accepting towards the idea of voting rights for freedmen towards the end of his life, it still only extended towards veterans and those white society seemed to be exceptionally intelligent.

We should also keep in mind that Marx did not necessarily believe that slavery's extinction in the United States was inevitable. One great fear of his was that it would somehow be incorporated into developing American capitalism, maybe along the lines of George Fitzhugh, or something. It's one reason why he supported Lincoln during the Civil War.

In all, I somehow feel kinda relieved we had Lincoln, when you compare his flexible approach to some of the completely inflexible white supremacists that came both immediately before (James Buchanan) and after him (Andrew Johnson). Though I wish we had someone more radical than even that, someone who could pursue land redistribution for freedmen, or even scrap the old Constitution.

thank you