I wouldn't call it Machiavellian, but you're correct supposing that mainstream anti-slavery sentiment and policy in the north did not come from overwhelming love for black people. The north was overwhelmingly white in terms of population, so it makes sense that they're not motivated by this altruistic, empathic urge we wish they were.

An important part of understanding slavery and the coming of the Civil War is knowing what Union and republican government meant to people in the late 18th/mid 19th century. The constitution stipulates that each state and the federation as a whole must uphold a republican form of government; it's right there in one of the founding documents of the country. A democratic republic requires virtuous citizens to function, and the idea of the virtuous citizen is tied to the then dominant form of economic organization, the small family farm. Because these small farmers were considered self sufficient, they could vote on principle, rather than being compelled to vote however pleased their economic masters.

This is where, at least according to one formulation, Northern and Southern political worldviews diverge. The virtuous citizen in the Northern conception becomes increasingly atomized, whereas the southern citizen is conceived as the head of the ancient patriarchal household, which included slaves as dependents. To the north, though, the conspicuously stratified structure of southern society, with a tiny planter class on top, followed by small slaveholders, the plain folk, the poor whites, the free blacks, and the slaves on bottom, appeared patently undemocratic and anti republican.

The differing understandings of society and government (as well as the material self interest of the South as a whole, but your question is framing the North as protagonists) really come crashing together with the acquisition of territory from Mexico. The Civil War often gets framed as an almost glacial conflict between 'Agriculture' and 'Industry', as if those were discreet Things that could act of their own accord. Kansas and Nebraska weren't exactly primed to become industrial powerhouses, let me tell you. Indeed, part of the appeal of acquiring territory in the west was the deurbanization and industrialization of the country. With western territories, second and third sons who couldn't inherit the family farm, and would otherwise get sucked into the maw of industrial cities, could instead head west to establish their own family farms, and thus prevent the increasingly hierarchal organization of the longer settled regions from crushing the independent republican spirit.

The threat of slavery's expansion into territories mobilized various northern political factions into a single issue coalition party, the Republicans, dedicated to stopping the expansion of slavery. Among their ranks were anti slavery Whigs, Free Soil Democrats, and Nativists. Sometimes, names are important, the shared commitment of these groups was to republican government. Lincoln spelled out what the party's single issue meant to them.

"Whether slavery shall go into Nebraska, or other new territories, is not a matter of exclusive concern to the people who may go there. The whole nation is interested that the best use shall be made of these territories. We want them for the homes of free white people. This they cannot be, to any considerable extent, if slavery shall be planted within them. Slave States are places for poor white people to remove FROM; not to remove TO. New free States are the places for poor people to go to and better their condition. For this use, the nation needs these territories."

Lincoln is an interesting case, though, in that he seemed to understand that he had to say things he didn't necessarily believe, and to not say things he did believe, in order to get elected. Principle only gets you so much if you lose, and leaving the government in the hands of the Slave Power -that cabal of planters and their cronies in the Senate, the Supreme Court, the Cabinet, and so on- was not acceptable. So he didn't rebuff the Know-Nothings, even though he disagreed with their principles. He also obscured his position on racial equality; while he considered black people as part of the Declaration of Independence (thus recognizing their right of revolution, which set off alarm bells for the South), his appeal on the campaign was aimed squarely at the self interest of northern whites.

To illustrate his problem, before the Civil War, about 70% of voters in Lincoln's home state of Illinois decided not to allow black immigration to the state. Assuming the 30% who voted Yes were all Republicans, that would mean that for Lincoln to win, he'd have to incorporate nearly a third of the racist voters who don't want anything to do with black people into his party.

This isn't to say all opposition to slavery was the acts of white nationalists (as opposed to white supremacists in the South), but it's not simply a case of Abolitionists and Fire Eaters arguing about the issues the way we wished they did. What I find especially interesting is the memory of this period. While the South's way of life has been endlessly romanticized, and tied up with the Lost Cause, and our modern industrial society has been projected onto the victorious North, many aspects of the South are still with us. White supremacy, conspicuous class stratification, economic bubbles, commodity fetishism, etc are alive and well. In this sense, a republic of virtuous small citizen-farmers seems to be the real Lost Cause.