was Ethel scared or stupid?
In the movie, Ethel makes a comment to Christie " i know where you should be", or something like that.I think that implies she at the very least had some suspitions about the deaths and Christie's covering it up ( or maybe that he had done them).
This no doubt alerted Christie, to the fact, she suspected him .As a result of this "she had to be silenced" so Christie murdered her.
So why do you think she stayed at 10 Rillington Place so long?.Especially if she had family she could have fled to, ( christie mentions Ethels sister later to cover up her dissaperance ?).Surly Ethel could have escaped him earlier, and reported Christie when she was safely out of his reach?
Perhaps this is explained better in the book (which i have not read)?.