In North America, bats most commonly have owls, red-tailed hawks, snakes, raccoons, and even foxes as their predators. Cats also will hunt bats if given the opportunity. You could even add humans to the list. While people do not kill bats to eat them in the United States, fear of these creatures cause many bats to be needlessly killed by humans.
Outside of the United States, there are places where fruit bats are killed and eaten by humans. Bat scientists are actively trying to educate these people on the importance of these bats to their ecosystem so that they would stop this practice.
In South America, and possibly other places, there are bat species that will predate on other bats. These carnivorous bats eat many different types of small mammals, and therefore will also eat bats that are smaller than them.
In North America, it is believed that the Hoary bat will eat smaller bats. It is well researched that the female bats of this species migrate to the northern parts of the U.S. to give birth and rear their young, while the males stay in the south and southwestern parts of the U.S. The reason for this is believed to be to protect the young pups from being eaten by the males of this species.
All in all, humans who do not understand the important role they play in nature kill the largest numbers of bats that die each year around the globe.