I think it's strange. I can understand maybe someone was mad about getting fired and took it out on the dad, but why kill the wife and take the kids? Maybe the kids just ran off or maybe the kids were the whole point.
I think that the killer was Danahy, Jessie. He must've plotted it to get revenge. Possibly, he committed suicide later but his body wasn't found. Now (1998) Doris is his daughter trying to get revenge on any alive Izards.