It also depends on what side of the planet you're talking about. In Eastern culture/mythology, dragons are generally considered to be wise, benevolent creatures. Western culture, conversely, usually portrays dragons as carnivorous killers, primarily due to the mythos of King Arthur and Camelot (which, ironically, was originally penned by a Frenchman). There was a British import, co-created by Monty Python's Terry Jones, titled Blazing Dragons, which turned the Camelot story on it's ear; the dragons were portrayed as the heroes, while knights were portrayed as the heavies.
As for dinosaurs, that depends on the particular species. Like Zoomie already said, it's usually only the T-Rexes that are portrayed in a negative light. Generally, dinosaurs are shown as being benevolent (however occasionally dim-witted) creatures.