I don't think it's necessarily as much about the gender of the perpetrator as it is her relationship (or lack of one) to the victim that makes people doubt she acted alone. Most women who kill in a rage kill their own family members, their own children, not their children's playmates. Most female sexual predators who kill are working in concert with a male predator. And most people trying to cover up an accident in a panic would involve others as accomplices out of necessity. It's not unheard of I'm sure, but it seems unlikely that it was a rage killing because MH wasn't related to Sandra, wasn't her caretaker, and had very little contact with her, so the next assumptions are that the murder was somehow sexual in nature or an accident of some kind. And in those scenarios, it's more likely that others were involved. Of course it's not impossible she acted alone, just that I think it's less about her being a woman and more about her being a woman unrelated to Sandra.