Actually, it was the slaveocracy that made it obvious that secession was about money, their perception of a threat to the $4 billion of capital invested in human flesh. The irony, of course, is that their actions precipitated the destruction of this capital.
Until the 13th Amendment was ratified after the war, slavery was legal in the US, and every Northern state agreed that it was legal when they ratified the Constitution. The North had sold the vast majority of their slaves to the South, pocketing the cash. A few abolutionists adopted slavery as a moral issue (both northern and southern), while the vast majority of northerners advocated separatist policies (aka "Free-soilers"). SCOTUS had issued a 7-2 ruling that slaves could be carried into the territories. Northerners refused to return escaping slaves - a part of the Constitution that they had agreed to. The yankees were the ones that broke the agreement.