why is Richelieu always so demonised?

I hate to burst anyones bubble here but the real Cardinal Richelieu bore no resemblance to how he is depicted in Dumas' work and the countless films derived from it. He was actually a very effective administrator of France and as pretty much the worlds first Prime Minister demonstrated a shrewdness and prudence that helped France grow strong as a nation. Not only did he help centralise French power, adding cohesion to the French nation-state, he also helped curtail the power of the Habsburg empire and though a Catholic himself promoted religious tolerance in order to create political unity. He also removed much of the Feudal power possessed by a number of the Nobility in order to decrease the risk of factional rebellion by the Barons. His greatest success though was as a War commander, with Victory at La Rochelle against Britain being a particular highlight. He was also a keen patron of the arts and education, setting up the Academie Francais and rebuilding the Sorbonne.

His negative reputation is perhaps because of the brutal and heavy tax levies he placed upon the Third Estate (though in fairness not any worse than any other ruler at the time), but he was certainly no traitor and the view he was derives almost entirely from Dumas who,as a post revolutionary thinker, turned Richelieu into a bogeyman for the First Estate, symbolic of the excess and corruption of the Ancien Regime. While the Ancien Regime was repressive, sadistic and corrupt, it is actually quite unfair that Richelieu was picked over Louis XIII, or his successor Cardinal Mazarin, as he was probably one the most progressive of the pre-1789 chief ministers; the best of a bad bunch so to speak.

In terms of his fate, Richelieu was not pushed into a lake or placed on trial for treason or anything rediculous like that as the movies show, but merely fell sick and died at home in his bed. His last words were to the King, to whom he said: "I have the consolation of leaving your kingdom in the highest degree of glory and of reputation.", and in many ways it was he that laid the foundations for what was arguably the greatest pre-revolutionary period, that of the Sun King Louis XIV.