Within the Book of Exodus, the second book of the Pentateuch, God sends the Angel of Death to kill every firstborn male child throughout Egypt. It is only those who smeared the lambs blood on the doors who the Angel of Death passed over. That's also why the Jews call Passover "passover". To celebrate when God saved them from death. <span />
I'd say there are two answers here - she must obey her husband's wishes and follow his advice (he tells her not to teach Douglass how to read) and slavery strips her of her innate beliefs and benevolent nature (she used to be a kind and caring person before she got slaves).