To add on to some of the suggestions in the earlier post citing Douglass's narrative, consider also the damaging psychological effects that the institution of slavery had on people. When Douglass makes his initial attempt to escape with a group of other slaves from his plantation, the authorities are notified and the plan is botched. Slaves suffered from extreme fear, and Douglass explores various manifestations of fear throughout his narrative.
I think that you might be better off hunting through the work to find the exact passages. I would pay attention to some of the portions where Douglass describes the forcible and deliberate breaking up of families. This might be an example of emotional cruelty or violence. Another example would be the transformation of Mrs. Auld, a woman who started off as good, but ended up being corrupted by the corrosive nature of slavery. Paying attention to how Douglass describes her transformation and how slavery caused it could be another example of his description of emotional violence. The calling of Douglass' work to understand how African- Americans subjected to slavery were human beings also could represent another example of calling out against emotional cruelty.
Below is a link to Douglass's Narrative.
Slavery is unjust and wrong simply because it violates the rights of freedom of the slave. This may be further accompanied by many kind of physical, economical and mental abuses of slaves that cannot be justified any of the moral or religious codes widely accepted in the world. However in addition to the injustice to the slaves, the slavery has many evil influences on the whole society.
Slavery, to begin with, strikes at the root of the idea of equality of men. Once the society stops believing in general principle of equality and justice, the conflict for gaining benefit for oneself at the cost of others is sure to extend much beyond to take advantage of only slaves. The existence of slavery makes insensitive toward freedom and right of others and in this way promotes disharmony in the world.
Slavery promotes arrogance and insensitivity among the slave owners. This kind of behavior is usually not limited to interaction with slaves. In this way slavery turns the masters of slaves in ineffective human beings.
Slave masters once used to accept slavery in any form. have reduced ability to fight against slavery or domination by others. Thus slave masters are likely to show reduced resistance to attacks on their own liberty and honour by other powerful people.
Slave are justified by the owners of slaves on the grounds of their need for labour or manpower. But now it has been well established that slavery is a very infective way of getting the best output from the people. Thus slaves are not even an economic means of getting people to do work.
My last point is controversial. But I believe in it so I will include it here. Slavery leads to reduces the happiness and mental peace of the slave owners because of their reduced ability to appreciate and receive the joy of giving service and being useful to others.
I would also like to add, the fact that people changed. Douglass' second master was a nice woman who started teaching him how to read, but she slowly became more cruel and was no long the same person.
Also slaves were kept away from any type of affection. Douglass grew up in the same house as his siblings yet they might as well have been strangers cause they rarely talked. He was separated from his own mom and never got to see her as much, he never knew about his heritage either.