The unwritten rule of tabletop Role Playing Games (but can be applied elsewhere). It says:
"Don't be a dick."
It essentially means players should treat each friendly and with respect.
If someone intentionally does something inappropriate, or acts maliciously towards another player or players, he's breaking Rule 0.
Particular behavior breaking the rule may differ according to a game you are playing. For example, some RPGs are designed as competitive games, while others are cooperative. Behaviour common in the first type could be sometimes considered bad (breaking the Rule 0) in the latter.
In most games, killing another player's character without proper reason* qualifies as breaking Rule 0.
* other than "I wanted your loot / I wanted to earn XP by killing you / I did it for the lulz" etc...