Answer: Rainsford ultimately has to decide if he will kill or be killed.
There are a lot of dark actions in this story, and the above quick synopsis highlights two of them: General Zaroff decides to hunt humans for sport, and Rainsford has to decide whether to kill Zaroff or get killed. The intentions and eotions, as you can imagine, are diverse: fear of death, anger at being hunted, the need to survive... there are more, but this will give you a start.
Explanation:
Because it is asking what Rainsford had to decide that was really dark
