The Rat King can be found in the tavern's cellar. Be sure to have the right cellar, since there are plenty of cellars in Hunter's Edge.
He'll only be there if he told you he'll be there. If he's not there, you'll have to lure him out.
Not that the Rat King matters anymore, since apparently you killed the wizard's servants hiding in the cellar.
I'm not going to ask why you killed the people hiding in the cellar. :S
You say you can't do anything more in wizard's house. That means you got in, right? Find an amulet in this house. It's what you need to progress further. After you get that amulet, you can basically leave Hunter's Edge.
