Actually, Bettas do not eat other fish, normally. They eat larvae, small worms, water fleas, etc... not adult fish. Your Tetras were probably repeatedly attacked before they died. And, I can bet anything that if your Betta killed the Tetras, it is not a matter of if he will kill your guppies - it's a matter of when.

Please give Mr. Betta his own, roomy tank. =) He is not suited to be around a bunch of male Guppies, you might as well throw him into a tank with other Bettas.