If most parents would do anything to protect their kids and cound never fathom deliberately hurting them, why are there those who direct their most intense violence against their own children?

It's a question that's been raised before in the high-profile cases of Susan Smith and Andrea Yates. And it's being raised again, following Monday's conviction of Barrie, Ont. woman Elaine Campione.

Campione, 35, was found guilty of two counts of first-degree murder in the deaths of her two young girls. The jury agreed with the Crown's assertion that Campione had drowned the girls in the family bathtub to prevent her ex-husband from being granted custody.

The Crown contended throughout the case that while Campione suffered from mental illness in which she had delusions that people were trying to steal her daughters and that she saw aliens, she also knew what she was doing at the time of the killings.

Campione committed the crime, the Crown insisted, out of hatred for her abusive ex-husband, whom she wanted to see suffer. They pointed to a videotape Campione appeared to have made within minutes of the murders.

"There, are you happy?" Campione asks in the tape, presumably made for her ex. "God's taking care of them now."

Dr. Sara West, an assistant professor of psychiatry at Case Western University School of Medicine in Cleveland, Ohio, says there are a number of reasons why parents kill their children. But spousal revenge is not very often one of them.

"This category is actually the most rare and is certainly shocking to the public," she told CTV's Canada AM Tuesday.

West points to the landmark 1969 research by Dr. Phillip Resnick on "filicide," the technical term for the murder of one's own child. Resnick found that there were five main motives in such killings, and the most common one was "altruistic."

In "altruistic" filicide, a parent kills her child because she doesn't want to leave the child in what she perceives is a "cruel" world. This type of murder is often accompanied with a suicide attempt. In a second type of altruistic filicide, a child is killed to end his or her real or imagined suffering.

Another all-too-common category is "fatal maltreatment," which is a death as the result of a beating, even though homicide is not always the goal.

(A subcategory of fatal maltreatment is "Munchausen syndrome by proxy," a very rare syndrome in which parents hurt their children in an attempt to get more attention for themselves.)

Another of Resnick's motives involves "unwanted child" filicide.

His final filicide motivation is "spouse revenge," in which parents kill their children to make their spouse suffer. West says it appears the Campione case involves this final category.

She says it's not uncommon for mothers who kill to have stressors in their lives, such as spousal abuse, but that doesn't mean there are warning signs for who will kill.

"There's no one defining characteristic of parents who kill their children," she says.

David Perry, a former homicide investigator with the Toronto Police Service, says the majority of child murders he has investigated were deaths at the hands of the child's own parents.

Perry told CTV's Canada AM it certainly appeared to him that the Campione drowning deaths were deliberate.

"She did a lot of planning, a lot of thought went into this. Dressing the children after the murders, and then creating this horrible video for her husband -- clearly, she knew what she was doing," he said.

Perry says he's investigated a lot of cases of filicide and sadly, in the majority of cases, the parents were perfectly sane.

"In rare circumstances do you find that someone is criminally not responsible because they are so mentally ill that they don't understand the gravity of what they're doing. Those are pretty rare," he said.

Perry remembers one such case in which a woman strangled her infant son in front of the grandmother.

"But as soon as I met that woman, I clearly knew that she was suffering major mental health issues. I didn't have to be a psychiatrist to recognize that. But in most all of the other cases I've worked on, clearly there was a thought process involved," he said.