Idea: The Reason Ron went from true friend to Jealous prat was because an integral piece of himself was taken from him.
All of the current generation of Weasleys would have done well in Slytherin. They were to a man and er, woman cunning, ambitious, and willing to throw the rules aside when it was to their benefit. The only reason they weren't in Slytherin was that family tradition dictated that they be in Gryffindor.
That wasn't to say that the Weasleys were Dark, disloyal, or evil. The idea of say love potioning someone into marriage was abhorrent to the entire lot. It was just that they all had dreams above their lowly station, and the cunning and the drive to take them there.
It was because of his cunning and the fact that he was a chess prodigy that Ron Weasley had started to spot the pattern in his second year at Hogwarts. Looking at what was going on, he could see that someone was shifting a pawn to become a queen. A queen that could be sacrificed later in the game since there was already another queen on the board. A board on which he knew he was just another piece being moved around in order to protect the King which was the entire future of the Wizarding Race.
Seeing what was happening to Neville just about made Ron's blood boil even knowing what could be at stake.
The thing was, while Ron was somewhat mad about Quiddich, Chess had been his true passion, had been his life, much as Dragons had been for Charlie and the joke shop had been for the twins and you can't take that much of someone's soul, their very being, without some seriously negative side-effects.