Play Fairly
It's important to specify the difference between in-character and out-of-character fairness.
IC fairness is not a requirement. If you are playing a thief, and you steal 1000 bronze coins from another player, they cannot complain about it to the staff. There are no rules against player killing, stealing, and other forms of IC unfairness and criminality. Such things are considered in-character; it is perfectly realistic if you are playing a malicious and unfair person to perform malicious and unfair acts. However, expect the proper IC reactions and retaliations. (If you get caught doing such things, the local law enforcement will be rather displeased. That doesn’t mean you can’t try to avoid the law, of course...).
OOC fairness is a requirement. Being unfair on an OOC level involves things like being a twink, which means using bugs in the code to your advantage (it is your duty as a player to report bugs to the staff), or putting words in another player’s mouth. Especially do not do things like emote actions for other people, which is describing their reaction to something or presuming that they perform a certain action. They control their own actions entirely.