How does Islam reconcile being a religion of equality with its stance on female and male slaves?

Slavery was ancient and widespread through various means, such as conquest, debt, theft, and need. Islam came and yearned for freedom, so it closed all doors to enslavement except one, and opened many doors to liberation from bondage. It legislated the freeing of a slave as expiation for oaths, zihar (injurious assimilation of one's wife to an unmarriageable relative), and killing. It also mandated the liberation of umm walad (a slave woman who bore her master a child) upon the master's death, as well as the mudabbar (a slave whose freedom is promised upon the master's death). It obligated the complete freeing of a slave if part of him was freed, encouraged manumission, and mandated good treatment of slaves. In summary, Islam is a religion of freedom, equality, and justice.