They were defending themselves against invasion by the U.S. Army.
Unilateral succession is illegal. Those flying the confederate flag were not defending themselves, they were committing treason. Our Union is an indestructible one. It was safeguarded then by the U.S. Army who will continue to safeguard it as long as America endures.
It feels me with great sadness to think of the blood that was spilled, but at the end of the day Americans who take up arms against the U.S. Army are criminals and traitors. No American should fly the flag of those who have killed our soldiers.
To summarize: 1) South Carolina secedes. 2) South Carolina demands the Union remove its troops from their soil. 3) The Union refuses, garrisons a fortress on foreign soil, and attempts to resupply said garrison. 4) South Carolina offers an ultimatum--remove your soldiers or we will attack. 5) The union refuses to remove their troops from foreign soil. 6) South Carolina's militia attacks Ft. Sumter.
South Carolina's militia fired the first shots, but I'm not sure how you can say that they 'Initiated hostilities'. Occupying foreign soil has always been considered an act of war. History is written by the victors, I suppose.
After seceding those who were illegally occupying the land were the secessionists themselves. You cannot unilaterally seceded from the Union. Hundreds of thousands of my countrymen died in the Civil War to establish this principle; it is the law of the land and to imply otherwise is to dishonor their sacrifice.
No unilateral secession, period. That is what 350,000+ U.S. military personnel died for. If you believe it was legal for those folks to secede I suggest you renounce your U.S. citizenship and leave.
An actual moral reason not to fly the confederate flag on your home or your video game automobile would be because many black people interpret it as a racist symbol, and they might feel unloved and alienated.
A philosophical reason would be if you disagree with the sectionalist politics of it. That's why many groups opposed the introduction of the symbol into state flags in the past. That sounds more aligned with your reasons, but is a different reason. The act of flying Italian, German, Mexican, or British flags to celebrate one's ethnic heritage seems quite different than that of flying the confederate flag.
Your arguments are transparently shallow, poorly thought out, and profoundly un-American. If my arguments would satisfy an automated grading system, yours would fail immediately upon parsing. I see no value in continuing a conversation with someone who claims there is no moral foundation in opposing the killing of U.S. troops by illegal insurgents. Feel free to have the last word here on this message board; in the real world the last word was the surrender of the Confederacy.