“The legality of secession was not seriously debated. What happened was the side with the most people and guns subjugated the other people who wanted out.”
That’s a fact. The federal government did not want to lose the revenues it was collecting from the Southern states. In 1861 the LAST thing the federal government wanted was to lose the lucrative element of the Southern states, with their agricultural engine and their four-season ports. It still had California, but it was too far away, still relatively undeveloped, and the Panama Canal was still forty years in the future). The fact is that in 1861 the federal government needed the Southern states far more than the Southern states needed the federal government.
This is true, but it was worse than that. The revenues produced by the South helped enrich the North as well, and if the South had been able to maintain their independence, hundreds of millions of dollars that went through the port of New York would end up in Southern industries instead, thereby creating a potentially serious financial competition for Northern Industries.
The potential prosperity of the South would have changed the economic world of New York, and it was a change that the movers and shakers of the North East did not want.
The South also had the potential to attract the not yet formed Midwestern states into their confederacy, and possibly even convince more states to join them, thereby weakening Washington DC even further.
An independent South represented myriad financial threats to Northern Interests as well as a massive revenue loss to the Federal government.
From the perspective of the North East, the South "Needed killin."