Why does Douglass say that few white people in Baltimore dared to publicly show humanity toward a black person?

Showing humanity toward black slaves is a sign to most people of weakness.  Even people who were not accustomed to slavery or who, like Thomas Auld, only acquired slaves through marriage and who thus might initially think slavery is wrong do not usually go against the prevailing mentality of the society.  Society even in Baltimore believes overall that slavery is just the way it should be.  Most people are not willing to go against society's values, regardless of what they personally think.  And some, like Thomas Auld, actually "overdo" their devotion to the tenets of slavery in an attempt to show they understand.how slaves are supposed to be treated.