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""Some people would get a psychotic depression and kill their own children because they're convinced they're doing the right thing," he added. "But frequently, those killings are not going to be as brutal (as the Groene killings) because, fundamentally, they care about the child."
In some odd way, he said, "They think they're being a good parent. They think they're putting the child in a better place."
Other than that, Worst said, it's hard to pinpoint the kind of person who could repeatedly smash in the head of a 13-year-old boy.
"Numerous types of people are capable of this kind of murder," he said, "some of whom are not mentally ill and in some cases are not even anti-social personalities. Terrorists are a good example of that. We're seeing that in Iraq."