When the various states volunteered to be a part of the Union we call the United states did they retain their right to secede from that union?
Would a war to stop the secession be just or unjust?
Is an unjust war an example of mass murder?
If the answers to those there are: "yes", "unjust", and "yes" then Lincoln would be a mass murderer for the unjust killing of confederate soldiers.
I think that of the three the last one is the weakest. Killing soldiers in a declared war even an unjust one is probably not murder. It would be a different kind of injustice.
1. Yes.
2. Unjust. More significantly, unnecessary.
3. Wrong question really. Adn it was the killing of all those Americans on either side of the Mason Dixon line in uniform and out.