I see 4 faults here.
1) As the issuer of the initial request to duel, it was Datruth's fault for not not stating what type of duel it was to be before sending the request. (If it would have been to me, I would have denied it right away simply for that reason. the whole point of the request to duel and trade stuff is that you should ask in character before sending the request.) As such, a "free for all" type duel is default, which Datruth should have realised. Fault one is that Datruth did not realise this. As it has been stated, don't request to duel unless you know what you're getting into. Getting upset about the oponents tactics in said duel when no defined rules have been paved out, only makes you yourself look bad.
And this doesn't even take into the account of what happens if you were to duel a dishonset character that would agree to the terms of a duel and then simply ignore said rules once the duel began. It would all be in character, as long as that is the type of character they play. Dishonest characters don't have to follow the rules they agree to. That's why they are dishonest.
2) The fault by him was when he got mad at Dutruth for using a potion. A free for all is a free for all. Using a game mechanic is just that. As it stands right now, that means you can chug 100s of potions with a push of a button. It won't be like that forever. Use the game as it is, adjust strategies when things change. Come into the forums and state how things are now, and how you would like them changed for the future. Then wait until things change and use the current system as it was designed to be used.
3) When the magic user ran away, Datruth did not run after them. They assumed something about the magic user. There is a wise saying about assuming things. Since it has a bad word in it so to speak, I won't mention it here. Point being, when you assume things, you run the risk of things happening you hadn't planned on. Then you must learn to deal with those unexpected happenings. Much of a man's character can be revieled by the way the handle unexpected events.
4) And finally. This is a freaking game! Why do people get so worked up over something so pointless? Oh darn, your DP might have gone down if you had lost. Guess what? DP are meaningless. 32 bytes of data on a server database. They won't buy you an all expense paid trip to the Carribean, nor a ticket to the international space station, nor even a loaf of bread at the store. They have no meaning. They are just another number. They aren't even used for anything in game. Getting worked up over a game is the biggest fault of them all. Roll with the punches, react to what gets thrown in your face, and try to overcome the obstacle. Don't go to a forum and shout how you felt your duel with "sir random player guy" didn't go as planned and then declare that there are rules to how all duels must be done and you are the duel king and thus your rules are absolute. (That was my impression you were giving off after I read your first post.)
This question has been asked before in this thread and numerous ones before it: Why is it that people must automatically cry DUEL! just becuase some random joe called them a name. "Sticks and stones make break my bones, but words may never hurt me" anyone?
Disclaimer: if it sounds like I'm trying to state my points of view as law, it is not meant that way. The above is simply my interpretations of what has been said and these words are simply my view point on the whole subject at hand. Nothing more.