It's pretty simple, really. Rainsford knows he has few choices. He can remain on the island, with Zaroff and the dogs at his heels, knowing that he has little chance of defeating them without a weapon (he has already used his knife in one of his traps). It means certain death. He can leap from the cliff into the dangerous waters below, where he might be killed on the rocks; or, he might survive the plunge, and then where will he go? Drowning is then a distinct likelihood. When he made the decision to double back and return to Zaroff's bedroom is unknown, but at some point, Rainsford added this possibility into the overall equation, and it proved to be the one option that he had to survive the night.