I think the whole reason that he didn't act against the Husnock ship that attacked the colony was a vow not to use his power, which promptly got forgotten when his *wife* was killed and it became personal. Which is a sort of understandable motivation, albeit a very weak one. It also has to be said that the power to wipe out an entire species with a thought is, as far as we know, even beyond what the Q can do--to all intents and purposes the guy here is God, and that's just silly.

Oh, and for extra fun, see if you can count how many times Worf says the shields have failed when the Enterprise is being attacked by the fake Husnock warship...