What reasoning did lincoln use to support the emancipation proclamation?

Lincoln reasoned that slavery was morally repugnant, and he had the support of abolitionists and Republicans to abolish slavery.

Details.
President Lincoln moved cautiously in signing a presidential order which did not pass through congress in order to become law.
Consequently, President Lincoln called for a constitutional amendment which was enacted in 1865, and eliminated slavery.

African americans at the time just came out of slavery and needed money to provide for there families. so they joined for the money mainly.