Why does Douglass make the point that a slaveholder who has fathered a child is likely to be tougher on that child?

I think
that Douglass' primary motivation in pointing this point of view about the slaveholder's child
is to reflect the psychological destruction that is so intrinsic to slavery.  Douglass shows
slavery to be both bad and psychologically damaging to everyone.  On one hand, the child who has
a slaveowner father, yet is a slave is one who suffers intense psychological trauma.  Their
lives are in bondage, yet they were brought into this world by one parent who is not a slave. 
This creates a divided consciousness, which, on many levels, is destructive to one's psyche. 
Douglass points this out in bringing out more opposition to the nature of slavery as one that
yields destruction and suffering on a personal level, something not as readily evident by the
outsider.  Additionally, Douglass brings out the psychological confusion of the master. 
Douglass illuminates how emotionally discombobulated and fragmented the slaveowner is.  On one
hand, they fathered a child out of their own choice.  Yet, their own self- hatred is what
compels them to mistreat this child and treat them even worse.  In this, the slave owners are
shown to be as psychologically fragmented, if not more, than the slave.