The word "providence" refers to God, or in this case to fate or luck. So what the story is saying is that usually it is luck that brings sailors to Zaroff's island. They get shipwrecked just by luck.
Zaroff says that when providence is not kind (when not enough sailors get shipwrecked just by luck) he helps providence along. In other words, he doesn't just leave it to luck. Instead, he also sets up that light out in the ocean. Sailors will think it shows a channel, but instead it just leads them onto rocks where they get wrecked.