Is the ending conclusive or sort of open? Why? What do you think happens next?

The end really isn't inconclusive. We know that Zaroff is dead because Rainsford is sleeping in his bed. Exactly what he said he would do. We can infer that Rainsford leaves the island as soon as possible, and hopefully, he sets free the men who've been imprisoned.

We'll visit my training school," smiled the general. "It's in the cellar. I have about a dozen pupils down there now. They're from the Spanish bark San Lucar that had the bad luck to go on the rocks out there. A very inferior lot, I regret to say. Poor specimens and more accustomed to the deck than to the jungle." He raised his hand, and Ivan, who served as waiter, brought thick Turkish coffee. Rainsford, with an effort, held his tongue in check.