Personally, I've always thought that Meta Knight had altruistic reasons for trying to take over Dream Land in Revenge of Meta Knight. Meta Knight makes reference to Dream Land's "lazy lifestyle"; perhaps he is trying to cure Dream Land of some perceived lethargy, perhaps brought about by its peace and tranquility. Though it wouldn't exactly explain why he seems to hate Kirby so much in that game.
For a while, the Kirby fansites were saying that Meta Knight had been possessed by the Nightmare, but that's been disproven. The rumour apparently came about by mistranslating a HAL representative's remark.