Where the Confederate soldiers terrorists?
First of all, any "soldier" serving a valid nation with a uniform and flag, can not be a terrorist. They may terrorize people, and they may even do the same things as terrorists do, but by definition, they are NOT terrorists. The term "terrorist" applies to persons who are not affiliated with a national military, who are attempting to affect political change through terrorist acts. Clearly, Confederate soldiers belonged to the militia of the CSA, a nation which had declared independence from the United States. Clearly, the CSA was not seeking political change, they were seeking independence and sovereignty.
Were the Confederates traitors?
No. This is established as natural law by our founding document, the Declaration of Independence:
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
This paragraph establishes, as a matter of natural law, that people have the right to declare independence over a government that no longer represents the people. Not only does it say they have the "right" to do this, it says they are "required" to declare the causes. This is precisely what the CSA did, and it doesn't violate the law, it follows the natural law as established in the Declaration of Independence.
So they weren't traitors and they weren't terrorists, they were people who formed a new nation and declared their independence from the United States. The soldiers were simply men who happened to live within the borders of the new nation, they had little to do with the decision and really didn't have much choice in the matter. Regardless of how any of them felt about slavery, they almost universally shared the objective of protecting their homes and land.
SLAVERY: It is important to note, the issue of abolishing slavery was not raised until well after the war started. Before the war, Lincoln had made several proposals regarding slavery, including a plan which wouldn't have outlawed slavery until 1911. Another brilliant idea of Lincoln, was to repatriate former slaves to other parts of the world, and he even had Congress authorize the purchase of lands in Central America, where freed slaves were actually sent for a while. Lincoln stated, in his debates with Douglass, that he didn't believe the negro would ever 'hold station' with whites in society. Lincoln thought it was preposterous to consider a black person equal to a white person. His Emancipation Proclamation only freed slaves in the Southern states, which were (at the time) out of his jurisdiction, because they had declared independence from the US. It wasn't until the 13th and 14th Amendments were ratified, that slavery was abolished in America.
Nowadays, we like to look back on the Civil War and say it was about slavery, but that is intellectually dishonest. The issue of whether or not we should enslave other humans, had been under debate for decades. Years before the war, the US had outlawed slave markets, there were no more slave ships, that had stopped completely. With the advent of the cotton gin and other technology, it was clear to see, the days of manual field labor were coming to an end. There was no resentment or animosity toward this from the cotton growing plantation owners, they welcomed new technology, and looked forward to the day they wouldn't have to maintain slaves. The real ISSUE was a matter of principle, a matter of law, and a matter of government. It was the US Courts (not the CSA) who had determined slaves to be "property" of those who owned them. Southerners didn't make this outrageous claim, it wasn't a matter of opinion, or subject to interpretation, it was what the court had decided and ruled to be the law of the land. According to US Courts, slaves were legitimate property. Now, that being said, the Constitution (4th Amendment) prohibits illegal seizure of your property. So the idea, in 1860, that the US government could come take property from individuals without cause, was the primary issue which compelled the separation. It would be no different than today, if the government up and decided they could come take your home.