Texas believed Lincoln’s election meant the end of slavery, so they seceded to protect it.
Some people argue the war was about states’ rights, but it came down to only one right: The right to hold human beings as slaves. States’ rights rhetoric emerged as an excuse after the war.
Southerners in 1860 were convinced that election of Lincoln gave them no hope. They believed he was out to destroy slavery, which their entire society was based on. This was true even for southerners who were too poor to own slaves themselves. This was true even as Lincoln continued to argue that he would not abolish slavery, but would only act to keep slavery from spreading to new western territories and to keep the Union together.
It is absolutely inconceivable that the U.S. would have gone to Civil War in 1861 without the conflict over slavery between free states & slave states.