It is incomplete to say that some people achieve happiness by making other people unhappy. (Expressed another way, to love to hate is not a form of love.) It is more precisely correct to say that some people enjoy a momentary feeling of power and/or importance by making other people unhappy. Many psychologists working to help such folks move promptly to the issues of impotence, powerlessness and low self-esteem which underpin this sort of behaviour, rather than dwelling on the behaviour itself.