Because if he has declared the slaves free too early in the war, the border stats which had slavery but never left the union such as Maryland and Kentucky might have decided to join the confederacy as well. He couldn't lose anymore support for the war effort
During Mao Zedong's rule, everyone who opposed the regime was deemed to be an imperialist and a traitor, and any opposition would be suppressed. People would get imprisoned or sent to work camps.
