By no Pk I mean that you shouldn't even think about killing someone for no reason. Doing this will get you kicked from the guild near imediately.
Pking is defined as holding shift/ctrl and attacking another player,
Pvp (duels), GvG, nation war, and war maps are all fair game.
Why?
It makes the guild look bad, and takes away valuble honor you need to get better stacks of potions among other things. They were swearing and cussing at me.
Any monkey with a keyboard can put a period in a swear word, for this to even be a valid reason to pk, the person would have to come up with something so amazingly insulting you'd want to rip them apart through the screen. So no, don't pk because of this.
They were KSing me and wouldn't stop even if I asked.
My first advice is to switch servers, but if you don't want to do that, your second option is to call in someone from the guild to intimidate them. If no one can come, you can't move to a different spot, and you find it amazingly difficult to just switch servers. Pk them.
They killed someone in front of me, I want to help!
Here we enter the area of grey, because of guilds and other connections even little noobie pkers can call in high levels to lay down the law on your face. And the people in our guild can't be expected to protect you because of your own misjudgement. We aren't the police, but to be fair.
You can pk someone who pked in front of you if, and only if their victim had no guild, otherwise let the other guild take care of it, so we don't have to get involved in anything too messy.
If their guild pked me in the past
Don't blame them for what their guild did.
If they pked me in the past
Fine, but don't go overboard and screw up your honor too bad. Though I'd like to point out, you lose nothing for being pked. They however lose honor. Let it go, and they end up losing honor while you lose nothing. If you decide to kill them back, you both lose honor.
They're pking me right now
Don't stand there! Fight! You lose absolutely nothing if you kill someone killing you.