Not to get into a political debate, but secession isn't treason. It was very much a fundamental right of the states to do so as a means of keeping federal power in check.
While I dispute the legality of this statement (and in fact, it is contrary to SCOTUS rulings... in 1868), that's irrelevant. Armed insurrection IS treason however you define it. As soon as the South took up arms against the federal government, they were actively committing treason. When South Carolina fired on the US Army trying to resupply a federal fort, they were taking up arms against the US government.
To your other point, Lincoln was not anti-slavery to begin with. He had slaves pretty Much up until the Emancipation Proclimation IIRC.
You don't RC. Lincoln did not own slaves. His family was poor and even if he was pro-slavery, which he wasn't, he couldn't have afforded them. He also rose to national prominence during the Lincoln-Douglas debates in 1858, in which a key issue was slavery. Lincoln took the anti-slavery position and argued against the recent Dred Scot decision. Lincoln also made his famed "half-slave, half-free" speech before he was president.
The idea that Lincoln was not anti-slavery does not withstand the barest scrutiny. Now, was he a radical abolitionist? No. Among Republicans, he was seen as a moderate. However, the Republicans were formed as an anti-slavery party.