I think a fish that eats another fish is called piscivorous. If he eats his own kind, he's a cannibalistic piscivore. In this case, he didn't eat it......he just mugged it. So your betta is a thug. My betta chases the neons and flagfish randomly, but he's just exercising his rule over the tank. Don't mess with his food, and don't mess with him while he's staring at himself, or you'll get chased.