Kenny was killed in every episode that he appeared in except for one, so he died many times that one of 'em had to take.
Although I agree that his departure was sudden.
South Park The Movie, I still feel was the pinnacle of show's success both comedically and commercially.
AS far as Tweak goes, I prefer him to Butters. Actually he's not scared so much as nervous. If you were nine and drank as much coffee as he did, you would be a wreck too.
