I believe he objected to the number two that preceded to the cardinal fish entry. Perhaps he simply doesn't like the number two. Or... perhaps he felt that a good welcome to his new neighbors was in order.
So... he ate one of them.
Edit - a few people have been commenting about betta's solitary nature. They can actually be raised very successfully in a community tank, just as long as they are the only betta/long-fin. The problem here was that the cardinal fish in question was way too small, and looked like a nice bite-sized treat.