I was looking for a motive... why did the killer want to hunt this poor woman?
The most satisfying explanation that I could come up with was that this man was a demon in human form - a manifestation/ symbol of guilt. As we learn, the woman was running away from this demon, but wherever she went, he was never far behind. She ultimately triumphs by defeating the demon. And we hope that she has resolved the guilty feelings she had connecting to her husband's suicide.
One thing of significance that I want to point out: we see in the movie that the woman married her late husband because he showed her a trick. The trick being referred to was a card trick but, the word trick is code for, essentially, a coward or wussy. So, the killer knew that the woman had a coward for a husband.
BUT, any speculations as to why the husband was a coward? What made him want to kill himself?