Why did the Confederate states believe they were justified in seceding?

The Confederates believed that they were following the spirit of the U.S. Declaration of Independence. In addition the Constitution does not expressly forbid secesion.

http://www.ushistory.org/declaration/document/index.htm

the confederates also thought that if they had a choice to be a united nation, they also had a choice to leave it.

The Confederate states believed they were justified in seceding for various reasons, primarily related to issues around states' rights, the institution of slavery, and political disagreements with the federal government. To understand their perspective more thoroughly, we can look at historical documents, speeches, and debates from that time period, particularly the secession ordinances and declarations of the Confederate states. These sources provide insights into their viewpoints and the specific justifications they put forth.