Going for one player at the cost of risking losing is not ruining the game for others. In fact, it is a valid diplomatic tactic. It happens a lot when instead of helping to set up a stalemate line, your "buddies" attack you, and you threaten to give the game to the leader. If they continue, I give the game to the leader, so they will know next time that I mean it.
The "problem" with diplomacy is the unpredictable nature of the players, so if one of them is stating that he is going to attack me no matter what, it actually makes the game easier, not only for me, but also for the other guy that is going to stab him.
Regarding Rule 9 - yes, we should have fun here, but not everybody have the same definition of fun, and not everybody think that losing is fun. I think that a lot of players who leave the game when they are losing, are doing so because they don't know how to lose and because they have lost the fun in playing the game. So, fun is not above all, and talking shit to another player is a valid diplomatic tactic (not that I would use, but it is valid). Inside the game everything goes, unless it is cheating. Going for one player, for whatever reason it is, is not cheating.