And I, in turn, appreciate your acknowledgement that the South seceded for the cause of slavery. Many Southern apologists still refuse to accept this fact.
But when you say, the North could have resolved the issue of slavery at less cost, I have to ask, at less cost to whom? How much longer should slaves have gone on suffering to avoid the costs of war? Weren’t they the ones paying the ultimate cost for further delay? And why was it the North’s responsibility to find a peaceful solution when the problem was largely in the South? On a more practical point, most Western nations slaved from afar. It is easier to let go of plantations across the ocean than when you have a majority of slaves in your midst. I suspect (but cannot prove, of course) that if America’s plantations had been largely in Africa or South America, it would have been easier to broker an end to slavery without civil war.
On New vs. Old Testament, you may have to take that up with evangelicals and other champions of “Judeo-Christian morality” who explicitly believe in the Bible, not just the New Testament. The only Christians I know who actually repudiate the Old Testament are Unitarians. More and more Christians are beginning to accept that homosexuality should not be a capital offense and alarming majorities of many majority Muslim countries still believe that it is. But demonising or physically attacking them makes no more sense and will have no more positive effect than punching neo-Nazis. To complete the parallel with slavery, it is fair to ask, “Given that they are paying the ultimate cost for delay, how long should homosexuals in Muslim countries have to suffer before others step in to end their plight by force?” I don’t know. As you said earlier, international and domestic action have (as they should) different rules of engagement. But I would be happy to begin the process of sanctions and diplomatic force right now.
Regarding your (rhetorical?) suggestion of war against slaving states, I would very much support an international front against ISIS and Boko Haram, backed by military intervention as a last resort. Some things can be chalked up to cultural differences. For me, slavery is not among them.