Joyce was an idiot (SPOILER)
At the end of the film, when the house is really going nuts, all the surviving characters are getting ready to leave, Steven tries to get her to come with her, but she INSISTS on staying.
Um...hello? The house has already killed a number of them, it's clearly dangerous at this point being in there, yet Joyce decides she wants to stay, all by herself no less. I don't care how obsessed she was about discovering and showing the world it's secret, this is much like leaping into the road just as a speeding semi comes by. A moment later when the ghosts come for her, she starts screaming and acting terrified, showing that she didn't want to die. Again, how could she do such a thing and expect not to be killed?