What tyranny specifically?
“Many Confederates viewed their insurrection as a second war of independence against tyranny.
What tyranny specifically?”
In its Declaration of Secession, South Carolina explicitly mentions the abrogation by fourteen states of Section 2, Article IV of the U.S. Constitution:
http://www.civil-war.net/pages/southcarolina_declaration.asp
This Constitutional crisis, and the election in 1860 of Lincoln, who was vilified by many in the slave holding states for his famous “house divided” remarks during and after the debates with Douglas, precipitated the series of successions.
Here I am trying to represent the common perspective of many in the South at the time, not to defend it.