Dear OBB readers; My fiancee treats her son badly

From a OBB reader

I'm engaged to a woman who has a three year old son. We are planning to get married by the last quarter of the year....but there is something she does that makes me cringe. She hates so much on her 3 year old son. At the slightest provocation, she spits venom on the little boy. She tells the child all the hurt his father caused her and even goes to the extent of saying he is a bad reminder of his father. I have spoken to her about this and she seems to be deep in this her ugly past and this is adversely affecting me and am thinking of walking out on the relationship. I don't want to marry a woman who is a bad mother.