That's just a type of bristleworm (some get huuuuuuuuuuuuuge!) which will feed on the already dead sea hare (or anything else), but they are not predators. I can't help you with the death of the hares though other than it's possible that they were close to starvation before you bought them, and there was no way to turn that around, or possibly you didn't acclimate them well enough.
