btw this is my opinion relating this whole story and Martin.
I am quite sure that if the roles were reversed... If it was an obviously evil guy winning against a quite good guy with rightful vengeance and the good dude was on the ground, Martin would never make the good guy somehow miraculously kill the evil guy with his last effort.
I think the whole Oberyn dying was something very un-Martin like. So far Martin was describing a very realistic, painfully realistic deaths. Like in normal world Ned would be executed. Everyone was expecting him to somehow escape but Martin makes this realistic and Ned dies. Same thing with Robb Stark. But now when the realistic thing would be for Oberyn to not act like a retard and finish this or at least make sure Mountain can't reach him any more, Martin decides that the villain will magically have the opportunity to kill the hero. I really am sure that he wouldn't write it that way in reverse situation.
Martin must just take pleasure from killing the decent guys.