Why is my soon-to-be-ex saying bad things about me to my child?
He or she may be saying bad things about you to the child for a number of reasons, such as insecurity, or hatred of you. The other parent may also be waging a campaign to turn the child against you and interfere with or destroy the relationship between you and your child.
This kind of alienation is, unfortunately, not terribly uncommon. If this behavior is taken to extremes it is generally referred to as 'parental alienation' or 'parental alienation syndrome' (PAS). A wealth of information on PAS can be found by doing a search for 'alienation'.