Dysphemism is the opposite of euphemism. That is, dysphemism seeks to express an idea in a way designed to be offensive - to make it sound worse. Death is an area where dysphemism is sometimes used - in armed forces in combat, for example, it can be a kind of humour designed to relieve the stress of seeing death constantly. In the R.A.F. during the second world war, for example, pilots were said 'to have bought the farm', meaning that they (and their aircraft) had dived into the ground - in other words that they had died. 'To buy it' derives from this, but is not dysphemistic, in that the physical image is no longer brought to mind by the phrase. Where 'friendly fire' is a euphemism for 'being killed (accidentally) by one's colleagues', 'murdered' is a dysphemism, implying that the mistake was in fact deliberate.
Dysphemism is sometimes used to mean 'hyperbole in a pejorative sense': exaggerating characteristics so as to make them sound worse. In this sense, you may come across examples of political 'dysphemism, such as calling anyone whose views are to the right of one's own a 'fascist'. True fascism is rare among the mainstream parties in the UK: strictly, it implied an extreme form of authoritarian government, in which the citizen was totally subject to the demands of the state, which was directed in violent forms of nationalism. Dysphemism is not, traditionally, the best term for this, but you may well see it: try 'pejoration' or 'abuse' instead, or even 'vulgar abuse'. (Colloquially, 'name-calling', or 'calling someone names', may be a better choice.)
Cacophemism seems to be a newer word to mean the same thing. In British academic English, use dysphemism for choice.