I didn't have a problem with this episode. The lack of development for the Husnock doesn't bother me because they're really not important - there's no shortage of aggressive species out there, and there's no reason to assume that Kevin's description of them is a complete or accurate one anyway. All that matters is that they attacked the colony, killed the others, and Kevin obliterated them in revenge.

It's true that his act of genocide is a larger display of power than anything we've seen from the other various godlike beings, but there's no indication that it's more than those beings are capable of, either. The very first time we meet Q, he's threatening the entire human race, after all.

I like this episode because it puts the Enterprise crew and the Federation in general in perspective - for all that we see them heroically running around the galaxy saving planets and defeating bad guys, there are just some beings that are so far out of their weight class that they can't even comprehend the implications of those beings' actions.