Of course, being a MYTH does not make it an untruth. In the circumstances it would be surprising if Jacques de Moulay et al did NOT formally curse those who had comdemned them. (What did they have to lose ?) It was not a classic Christian case of "Father forgive them, they know not what they do". The King and the Pope knew exactly what they were doing and the baseness of their motives was probably known to all.
What makes it surprising is that it appeared to bite!
Well, I won’t go into te political reasons either of them might have had, but everybody can see why a myth like this could start and fester. King and pope were dead shortly after the grandmaster was burned (why am I thinking of a plot? Must be the books I read these last years...) and people started talking; they surely knew why that was! One thing led to another and before you are one generation on there’s a pretty nifty explanation of why the ‘enemies’ of the Templars had died so soon. Of course the proper way of looking at this is asking yourself why the king and the pope both died in 1314.
According to the placque in Paris De Molay was burned in Paris on the Isle des Juifs on March 18, 1314, ca 60 years old. Pope Clement V, Bertrand de Got, died in Roquemare on April 20, 1314, just a month after De Molay. Having been born in 1264 was ca 50 years old. He is described as weak in both body and character. King Philip died November 29, 1314, at Fontainebleau, about 8 months after the grandmaster. He was born in 1268, so he was about 46 years old. Were they healthy men before they died, in other words: was their death unexpected? Or had they been ailing for some time? I don’t have the time nor the inclination to find this out, but if there are sources from which we may deduct the state of health of both men – and why wouldn’t there be? - this can be done. It is also do-able to find out when this curse thing first got written down. I suspect that this is typically a juicy bit for a chronicle (Grand Chroniques de France, Froissart?).
I do know that Maurice Druon made a meal of the prophecy in his series of 7 books called ‘Les Rois Maudits’ (1955-1977; wonderful read...) and I, again suspect, that these popular novels have a lot to do with the curse being so ingrained on the brains of everybody who’s a Templars fan. And boy are there a lot of those, nowadays...