https://en.wikipedia.org/wiki/History_of_slavery_in_Texas
So, perhaps, if we study the history of the Texas War for Independence, as I know we had a class on that in Middle School; one may see that part of the war for independence may have been to establish slavery as law.
Ah, I see now that your point is as much about the Texican's revolt from Mexico as about the War Between the States. Not being a native Texan I've never formally studied that war, but it is my understanding that among the Texican's aims was the establishment or perpetuation of slavery in Texas. Native Texans are of course free to correct me on that.
And slavery was certainly a key rationale for the secession of the seven states of the Deep South, including Texas. But it was not a rationale for war. The Emancipation Proclamation took effect January 1, 1863, some 21 months after the war began. Why was the Union fighting during those 21 months? To defeat secession, not to defeat slavery. And why was the Confederacy fighting? For the right to secede.
It is a great tragedy that both the Texicans and the Confederates made a self-contradictory argument about freedom - the freedom to enslave others. But what is clear to us today was not as clear to them. At some point in the future people might well argue that Americans of the 20th and 21st century were clearly hypocrites for arguing that a woman's "rights" included killing a baby in her womb; Dred Scott and Roe v. Wade might be discussed in the same paragraph of a law school textbook. We err in projecting our understanding and our beliefs back into the past and expecting long-dead men to conform to our standards.
And is the argument for liberty really defeated by the fact that people misuse liberty? Am I really only allowed liberty if I use it in conformance with someone else's standards? That would be no liberty at all.
So while I maintain a keen interest in the history of the question of a state's legitimate powers and rights versus those of the Federal Government, I reject the idea that prior history invalidates present rights. That Texans, in their fight against Mexico and their later fight against the US Federal Government, believed in slavery, has no bearing on the rights of Texans today, or the rights of the people of any other state, to govern themselves.