Continuity mistake: When Henry attacks Connie for coming into his room and Mark steps in to stop him, they start fighting and Connie runs downstairs to tell on them. As the shot alternates back and forth, Henry's right hand jumps from Mark's ear to grabbing a handful of his hair and then back down again, but we don't see either boy move in a way that would explain the switch.
Continuity mistake: When Henry & Mark were chased by the dog, look carefully at the back of Henry's jeans and you will see a large dirt stain on it, but in the following scene over the cliff, there is a brief overhead shot over Henry and you will see that stain is gone.
Continuity mistake: Mark and Henry are climbing up to the tree house. Mark is back on the ground looking up at Henry in one of the shots; then he is higher up.
Answer: Henry caused his younger brother's death by drowning, just like he was planning to kill his sister, Connie.
raywest ★