After understanding the issue of what a slave is you need to understand that the bible is first about saving souls, not about social justice. Jesus said let the dead bury their own dead. His mission was much more urgent than worrying about the economic systems, which would change once hearts and minds are changed.
The above on slavery in Galations should put Pauls views on slavery in perspective. Slavery was not prescribed but a sense of being a slave to sin was and still is vastly more important.
the argument in a sound bite is that a slave can know and love Christ in the world while an aborted baby cannot.
I believe that social justice is the outgrowth of practicing what Jesus calls us to--belief in His saving grace. If social justice becomes the goal, then we lose sight of Him. OTOH, if our society is not just, I would respectfully submit that we are not following what Jesus calls us to.