In the movie, The Grudge, near the end it is shown that-
the father of the Japanese family have killed the mother (by chasing and stabbing her?) and have drowned the black cat. He has done this in a fit of rage upon discovering the mother's affection for the professor. He then kills himself by hanging.
Throughout the movie, the little boy (Toshio) is visible to the characters who eventually die. And when the father is killing the cat, Toshio is seen hiding and cowering in a corner.
But how did Toshio actually die? (And did he?). As I remember, it is not shown clearly.