The NYPost editorial this morning is absolutely right:

By engaging in this sort of intemperate rhetoric, Matthews has effectively disqualified himself as an “honest broker” for this coming debate.
NBC needs to replace him as a debate moderator immediately. The job – by definition – requires an attempt to display objectivity going into an event.
Yes, the distinction between “objective journalist” and “opinion-maker” in recent years has been blurred to the point where they’re nearly indistinguishable. But Matthews’ comments obliterated any possible distinction.