Answer:
To add a visual that creates suspense.
The most likely reason that the movie adaption of "The Most Dangerous Game" changes this scene so that General Zaroff shoots arrows at Rainsford and his companion is to add a visual that creates suspense.
Explanation:
They had a reason to add a visual that creates suspense because it would be difficult to receive the same reaction as is created by reading the scene if they had portrayed that scene in a book.
So movie makers wanted viewers to keep having a question of what would happen next.