The north went to war to defend the union and ended up saving the slaves. The south went to war to save slavery and ended up losing everything.
Your logic does not hold.
The north went to war to secure a tier of states it could continue to treat as a second-class source of raw materials for its own mill industry. Threats to end slavery were threats of economic sanctions. Tariffs were designed to force trade with the North and hamper industrialization in the South to preserve the status quo. The cause was economic in nature.
False altruism might make for nice fairy tales, but it was all about the money.