"Most weren't abused?" Among those where it was known whether there was a history of abuse or not, more were abused than not. You only get "most weren't abused" if you assume that most of the not known cases were no abuse cases. Since presumably these murders were investigated, it's kind of strange that there are so many "not known" cases, and since they don't say what would put a case in the "not known" category, it seems unsafe to speculate about what the "not known" cases are like.