What are the rules? There are no rules aside from what the player makes for themselves. I don't attack flagged afk players and if somebody is at low health, I'll let them heal up this includes if they have multiple NPCs on them.
"Ganking" for me and I use that term loosely is more about having a fair fight as possible so I can learn more from it, I absolutely don't expect any of these "rules" that I set for myself to be enforced by other players though, so always be prepared!
So really, there are no rules, just players with differing play styles.