Dead Island isn't all about zombies in paradise. It's also about the people who need to survive the undead horde, and Sam B. is one of the unfortunate guests who will be fighting for his life.
According to Deep Silver's press release, Sam B. is a one-hit-wonder rap sensation who is at the island's hotel to perform his song "Who Do You Voodoo." Then a bunch of other stuff probably happens. The point is this: zombies are everywhere and Sam B. needs to re-kill them. You can see Sam in action in the screenshots below.