I think you paint a skewed version of history, slavery was alive and well in England until 1772 with the James Somersett's decision. We can argue over the semantics as to whether a serf is a slave by legal letter, but for most it is commonly accepted that the institution of slavery existed in England until 1772. It also existed in many colonies that existed under many structures of rule. It's existence, in England can be traced to Rome (it actually existed in a form even before Roman occupation), who can trace it to Greece who can trace it to Egypt who can trace it to Mesopotamia. In each of those empires, the rights of slaves ebbed and flowed just as today the rights of citizens ebb and flow. In England slaves had some rights, in the US they had less, in Haiti under the French even less, but it was all slavery and they can all trace their roots back to Rome and as such back to Mesopotamia. But to argue that what England had was not slavery in comparison to the US is akin to arguing what the US had was not slavery in comparison to Haiti. The reality is very different though, they where all indentured, and deprived of rights, to most that classifies as slavery. Roman slavery was also instrumental in shaping the western views on slavery (which was a pretty brutal view).
The republic is no more or less responsible for slavery than any other form of government, with maybe the exception of Fascism. Abolitionism and industrialization can take far more credit for the elimination of slavery than the structuring of a government. The fact that slavery was not abolished in Norther states until 4 years after the war started lends credence to the fact that the institution of slavery fared no better due to a strong central government in DC, what ended slavery was a combination of changing cultural norms (the abolitionist possessing the stronger moral argument) and Industrialization requiring the need for skilled and educated workers. These are the constructs that ended slavery not a strong central government. Hitler had a strong central government, and it could be argued that he enslaved millions. It is not the system of government but rather the morals and virtue of the culture.
in both cases the better side won
Yes in much the same way the better side won in the Iraqi war. Some good will come of it and some bad will as well. But with conflict there are always a host of agendas at play, boiling war down to "liberating the Iraqi people" does us a disservice and makes us far more susceptible to repeating past mistakes, because every war that has ever been fought has been justified by a moral cause. But morality does not emerge from acts of immorality. It emerges from the enlightenment of moral people such as the abolitionist, or Dr. King or Gandhi.
War is never morally virtuous, it is men killing men and usually for resources or power, forgetting that, allows us to succumb to moral propaganda, when we need to be vanguards of peace.