These Piranhas do have razor sharp, interlocking teeth, and they do sometimes have "feeding frenzies," where a school of them will consume prey and strip it to the bone. But their normal meals includes other fish, insects, crustaceans, and even algae! Don't get me wrong, Red Bellied Piranhas will sometimes feed on much larger creatures, but it's typically because they are sick or injured. In their native habitat, humans are not often bitten by Piranhas, and the Piranhas are often consumed as food!
And despite their aggressive reputation, Red Bellied Piranhas make pretty decent parents. The males and females will protect their nests, and then, after hatching, guard their brood of young for a period.