Must have been surprising to the Native Americans being slaughtered and sent to reservations and all the immigrant labor being exploited to make those railroads as well. America is the land of the free for the upper caste.
Founding Fathers, who were mostly abolitionists: "Slavery is surely a dying institution, we can just put it on the back burner and let it wither away, that way we can avoid a civil war."
In their meager defense, they were correct at the time, slavery was a dying institution. It wasn't until the later invention of the cotton gin that slavery exploded in profit. By then, it was too late. The economic interests of the slavers had grown and entrenched, and the war became inevitable.
Not to defend the slavers, or their advocates among the founders, just to explain the founders' reasoning a little more.
For further reading, the Wikipedia page for the Cotton Gin goes into some detail.
The invention of the cotton gin caused massive growth in the production of cotton in the United States, concentrated mostly in the South. Cotton production expanded from 750,000 bales in 1830 to 2.85 million bales in 1850. As a result, the region became even more dependent on plantations that used black slave labor, with plantation agriculture becoming the largest sector of its economy. While it took a single laborer about ten hours to separate a single pound of fiber from the seeds, a team of two or three slaves using a cotton gin could produce around fifty pounds of cotton in just one day. The number of slaves rose in concert with the increase in cotton production, increasing from around 700,000 in 1790 to around 3.2 million in 1850. The invention of the cotton gin led to increased demands for slave labor in the American South, reversing the economic decline that had occurred in the region during the late 18th century. The cotton gin thus "transformed cotton as a crop and the American South into the globe's first agricultural powerhouse".
I don't think the founding fathers realized the sheer agricultural potential of the country, especially if you compare the size of the country's territory to that of what it would be by the Civil War.
Then there's their backgrounds from the British, where an abolitionist movement had already existed for quite some time and the fact that the British were already industrializing and needed slavery even less. Great Britain was fairly liberal in general, so I'm sure that had a lot of influence on the founding father's viewpoints. For context, during the Final Act of the Congress of Vienna in 1815 Castlereagh the main British representative managed to sneak in a handful of statements on slavery, suggesting the other powers should eventually end the practice. This was more so to impress the British people due to the current on-goings of British politics, but it's still only 30ish years after American Independence.
They should have told the gentlemen from Georgia and Carolina to suck it, held the war without them, and proceeded to flatten both colonies once we kicked the Brits out of the other 11 colonies.
We missed starting the country as an abolishonist state because of two men, and one single vote. Had anyone else voted to abolish, we would have told Georgia and Carolina to shut up about it.
And you know the crown denying those rich land owners the opportunity to keep pressing west into indigenous lands. Which was imo the biggest factor in the revolution.
Shit was so bigoted back then they said that unironically and everyone immediately understood it only applied to rich white men who owned land.
Its not quite that simple. You had a lot of discourse on and around the concept of liberty, particularly in the major urban centers and universities. But like a great deal of revolutionary sentiment, the theory of liberty was precluded by the reality of economic incentives.
The Continental Army was funded by the Virginia plantation class largely thanks to their response to the Dunmore Proclamation. Northern territories more predisposed to emancipation were crippled by the merchantalist system of England and could not assert themselves financially or militarily until some 80 years after the American Revolution. Southern territories, which had rapidly depleted their agricultural soils thanks to aggressive tobacco harvesting and poor crop rotation strategies, were cash rich but labor poor.
So you had a marriage of convenience between plantation states and industry states for the purpose of kicking out British colonial military. But it wasn't as simple as saying "Everyone quietly agreed on slavery". The abolitionist movement existed during the colonial era, it simply didn't exist in the states with the economic and military supremacy... yet.