I've kept serpents and brittles for years with no problems with corals or fish. You DEFINITELY want to avoid the infamous green brittle star however:
O. Incrassata (and a couple of it's relatives) are excellent fishcatchers. I've never seen/heard of serpents or brittles trying to kill corals however. I have to wonder if the colt coral was dying or injured and the brittle was feeding on the dead/dying matter. They are scavengers, but IME, never bother the living.