Was the south justified in seceeding from the union?
They justiifed it by pointing out that the USA had seceded from the British Empire, and that the Constitution upheld States Rights.
The legal argument was, in fact, very complex. The Confederate President, Jefferson Davis, wrote two books of memoirs that consisted of nothing but dry legalities - quite unreadable, and a missed opportunity to inspire younger readers with the legend of the Lost Cause.
The South was, of course, claiming States Rights to practise slavery, when a small but respected minority in Congress were preaching Abolitionism. So really, the justification was more moral than legal.