If I've just gone through a long journey, hell-bent on revenge, this guy is still dying. The kid being there just makes in more interesting.
First, make sure the kid can't leave. Then, incapacitate this guy. Break his arms and legs or something, so he's in a great deal of pain, AND can't move. Then, say some obligatory evil dialogue. Then, brutally kill his kid right in from of him, in a really gruesome fashion. Then, more obligatory dialogue, THEN, kill this guy.