Bonnie is Herb's wife of 5 years. She's a reclusive, anxious person who's suffered from depression since the birth of her first child. Subsequent children brought more episodes of postpartum depression, for which there wasn't much treatment in the 1930s and 40s when she was having her children. As a result, she only functions well for short periods of time, and spent plenty of time in psychiatric hospitals. When the book opens, she's dreading Thanksgiving and all the company it will bring.
Bonnie can't stand having to make decisions during Herb's frequent business trips. She's more or less turned over the responsibility for running the household to her daughter and husband. But she loves her family, and she worries about her depression affecting them. She fears that her children will remember her "As a kind of a ghost […]" (1.91).
It's worth noting that the Clutters seem to have a secure marriage despite one of them suffering from depression for years on end. We know that Herb's sincere religious beliefs would prevent him from abandoning a sick wife. A love triangle as a murder motive is dismissed immediately as a possibility by the investigators.
At the time of the killings, Bonnie has been given reason to hope. She's been told that her depression is a result of a spinal problem, and that surgery can cure her. Still, we last see her alive going to bed, her Bible on her bedstead, with a bookmark that reads "Take ye heed: watch and pray. For ye know not when the time is."
Ye sure don't.