ONE EXPLANATION
«Negative», vengeful or just plain petty spirits of human origin had taken residence in the house. They wanted the house for themselves or just enjoyed causing others pain.
One or more family members were susceptible to it, and they worked through their possible hosts. A house can't be possessed, at least not as living beings can.
I suspect the spirits were christians themselves. Therefore they didn't take kindly to the priest attempting to exercising them. So they took on the «form» of what was «expected» of them.
I should point out again that I don't believe in god or satan or any devil or higher being, only living beings, in various forms.
So an exorcism doesn't work, unless the spirit is scared by it or chooses to leave. And these spirits were very determined, and yes, obviously malignant.
They «took» DeFoe first, him being most susceptible, and used the energy from the subsequent murders to grow even more powerful.
Oh, they have probably been in the house for a long time, perhaps centuries. Time, if time is at all moves differently in the spirit world. But something changed. Perhaps they just got bored, I don’t know. I wasn’t there. The Lutz family, however, was, and they suffered for it. I’m also inclined to believe the malignant forces later worked in tandem, either deliberate or coincidental with the establishment and Kaplan and various «hoax-busters» and debunkers to reestablish the ruling, present day viewpoint of reality, thereby increasing the Lutz family’s suffering and denying them any change of effective help.