Besides training Tom, and Jack, what does Mary Britten do for a living? What does she sell and what does that product do?
It is clear from this excellent novel that Mary Britten has a nice little "sideline" in pills that trigger miscarriages in pregnant women. This is clearly stated from Jack's flashbacks to when he was working for her, but also it is again referred to when Mary Oates, on Jack's instructions, goes to Mary Britten's house to purchase some of her tablets to trigger a miscarriage in her sister, who is bearing her husband's child. Note how Mary Britten "covers" herself from what was at the time an offense:
"These pills," said Mrs. Britten, indicating the illustration in her puff, "are from a recipe given me by a Swedish doctor. Very good they are....Very good pills... except for just one wee shortcoming... And the shortcoming is as follows: should you be so unfortunate as to take them when you was with child, then, oh dear.... Know what I mean?"
So, Mrs. Britten is still selling this product that she used whilst she was training Jack in his childhood, and was used to kill his child.