I've been asked quite a few times lately, which makes me wonder how much misinformation is out there.  You can kill anyone for any reason.  Its your gun, you built it, use it.

Someones name starts with a R, shoot them.  Someone said something on IRC that you don't like? Shoot them.  That guy logged in when you were feeling in a destructive mood?  Shoot them.

Sure there are ethics and crew/city rules involved that may contradict or even override this, but as far as the game goes, anyone for any reason is legit.
There are a few exceptions:

Your city, or family rules.
Bloodlining someone unprovoked many m any times to the point where it becomes harassment.
Shooting admin accounts or admin test characters.
Accounts with specific instructions in their profiles. (4hs, -48s, an so on)

Just making sure that is cleared up and out in the open.  You should only have to worry about in game consequences for using your gun, not admin consequences.