Assuming no trauma or abuse has been omitted from her story, it seems that the breaking up of her family was the catalyst for her leaving. Still, leaving because you don't like your step dad is so extreme. Speculation, but I'm thinking she was dealing with mental health issue's as early as the 80's when she left home. She may have felt that she just wasn't understood and didn't know what to do but leave.
Is this a woman who struggled to feel a sense of belonging anywhere for her entire life? Did she secretly go off and cry during the times she left family events, feeling lonely thinking about the family she left? It makes sense that she held her own baby so tightly, and I can see how the breaking up of her own marriage could be the catalyst for her breakdown/suicide. Having her own little family may have given her a sense of safety and belonging that she hadn't felt since childhood, and then it unravels, leaving her back where she started. Alone. I'm rambling and I know nothing, but my heart is so heavy for Kimberly right now.