That is a matter of opinion.
They did have the right to leave the union, it says so in the constitution.
Leave the southern states immediately
The southern states threatened to leave the Union because of antislavery talks.
It is secession
Leave the southern states immediately
The southern states made the assumption that Lincoln would make slavery illegal.
The Southern States believed that they had the right to secede because the Union had violated the US Constitution. Because of this the Southern States believed that the Republican dominated Congress and the views of President elect Lincoln would continue to do so, thus the Southern States believed that they had the right to leave the Union.
Secession.
Leave the southern states immediately
Leave the southern states immediately
Leave the southern states immediately (apex)
Leave the southern states immediately