Yeah, with Marilyn returning home from college, you wonder if she left behind a string of unsolved murders there. If the show had continued she may have turned out to be the most "evil" character on it.
I think Fuller hinted she was exactly that in some of the interviews.
In fact, if you think about it, that's actually a very logical conclusion. The rest of the family can't help what they are. They're monsters. One could argue they have to engage in at least some "monstrous" behavior to survive. Not so, Marilyn, however. She chooses to live among them, even after Grandpa suggested she might want to move away.
When you consider her role as the human face of the group, who finds them a home, takes Grandpa around the neighborhood, etc., she's basically a pretty Renfield.