Fort Sumter belonged to the federal government of the United States of America. The South Carolinians had no claim to it.
There is no “right of secession”, although Madison did remark that there were two ways that a state cold withdraw: the same way they came into the nation (through congress) or through rebellion.
There is every bit the right to leave the union, just like the right to murder the unborn. when the U.S. government is the oppressor we have a duty to resist. Btw once sc left the union the Feds should have left.