Honour, Code of Conduct, etc.

[Richard]

There hasn't been an official rule against that ever, you're free to attack/kill people at the FC any time you want.

True. Although a lot of people will probably consider it lame to do.
Myself including, although again depending on the situation / person If someone always shoots @ FC, I wouldn't hesitate to shoot him myself when HE doesn't expect it. Although I haven't really been there in ages