There are those of us who believe Lincoln saved as much as he could of the Constitution and rule of law from those intent on destroying them.
If you think Lincoln was particularly aggressive or unlawful in how he handled opposition, even treasonous opposition, you might want to research how similar issues have been handled in other great civil wars. The US WBTS had far and away the least oppression and fewest atrocities of all such wars in all history. (Which unfortunately isn’t saying all that much.)
It all depends on what you consider to be treason. There are many, myself included, who believe that there is no higher form of treason than for a politician to elevate himself above the Constitution. It was under Lincoln that this became possible marking the realization of Benjamin Franklin's warning about the Founders' bequest of a Republic "... if you can keep it."
But don't think me unfair to Lincoln or blind in my faith in the Founders. You can argue with some credibility that it was their failure to end slavery at the time of the founding that paved the way for Lincoln and his ilk to destroy the independence of the States a generation later. And the end of the sovereignty of the States was the end of limited government.