You can't post hoc justify an act you took. This is like shooting into a crowd and striking a murderer, and then claiming you were justified in shooting into a crowd because you killed a murderer.
You have to justify what you did on the basis of why you did it *WHEN* you did it. You cannot come along later and claim a new retroactive reason for why you invaded and killed other people.
Also, the president doesn't have the constitutional authority to declare an entire region in "rebellion" and then seize all their property. He is still required to adhere to "due process."
Besides your opinion, show me the case law, supported by court decisions that makes Lincoln’s actions illegal.
Actually the President did have the power to declare a whole are in rebellion. In fact he had the sole authority as decided by the Supreme Court in this decision;
Martin v. Mott
25 U.S. 19 (1827), affirmed the president’s right as commander in chief to call out the state militia. Complying with an order from President James Madison during the War of 1812, the New York governor called out some militia companies. Mott, a private in one of those companies, refused to obey the order; he was court-martialed and fined for this refusal. Martin, a U.S. Marshal, seized Mott’s property to enforce the judgment when Mott did not pay the fine. Mott brought a civil suit to recover his property. The Court held that the president had validly used his Article I power to call out the militia, that he had sole authority to decide whether or not a situation permitting use of the statutory power existed, and that this decision was “conclusive upon all other parties” (as the states). The case set a major precedent in support of President Abraham Lincoln’s decision to assemble troops in the cause of defending the national union.