I've been asked quite a few times what is the rule concerning killing someone else's character. While there isn't an official rule about it, there are some guidelines that we encourage people to observe, for the sake of keeping things friendly in the RP.
Killing another player's character is a situation you might find yourself into if your characters are inclined to fight, or if you are an assassin, or sometimes you're put in that situation unwillingly and must make a choice between letting an enemy live or striking them down.
Regardless of how you get there, killing another character does not require their consent, however, it is polite to ask them. Some people have characters they are very attached to, and are not willing to just let them die. Sometimes you can just agree to beat your enemy senseless and leave them near death, or something like that instead of outright killing them for good.
If you don't want to just beat the crap out of them and leave them for dead or near death, here is how you go about killing someone:
Is the character you want to kill in a situation where he undeniably could not avoid being killed?
Examples of that would be... when the character is unconscious or helpless in some way that he can't try to get away from a killing blow.
If the character you want to kill is not helpless, is he able to evade your attack and is he playing fair?
That last bit is very important: Many times when someone doesn't want to risk losing a character, they will resort to metagaming, godmoding, or simply disappearing from sight (TPing away). Sometimes a simple IM to your assailant asking him not to kill you will suffice to arrange for another outcome for the battle.
If your opponent is playing fair....
Are you playing fair?
Some people are truly capable of evading many forms of attacks. Many characters greatest power is their ability to defend themselves. If you have doubt that what they are doing is fair and that they can truly be avoiding your attacks to the point of frustrating you, ask them what kind of carded ability it is they are using, and if you still have doubts, ask a staff member to double-check that indeed they can do the things they are doing.
Ideally, in a perfect world, when two characters fight, the players will concede to their strengths and weaknesses, and reach a conclusion where it's either a stalemate, nobody dies, or where one of the characters ends up in pretty bad shape or killed.
Be fair!, whether you are attacking or defending. Acknowledge that you might not always win. If your opponent is giving you a good RP, they might have 'earned' that killing blow.
Now about that... You caught your opponent, cut off one of their legs, and left them to bleed to death.
Nope. Doesn't work like that.
Unless you give someone a definitely, undoubtedly killing strike (such as beheading them, or stabbing them through the heart, or cutting them in half, etc), don't assume they will die from their wounds. If you don't deal the killing blow, it's up to the other player to decide if they live or die. In the Fantasy RP setting, there are many ways to make someone who's on the verge of death get better.
We have a bunch of assassins around who can make some pretty strong poisons.
Usually we don't approve any poison or substance that can be administrated in a sneaky manner to have the ability to kill another character. The reason for that is that killing someone in that way does not promote RP interaction.
Poisons are usually advised to cause great pain or illness, but I can't think of any one that outright kills without giving the victim a chance to at least have someone in their behalf try to find a cure.
The fact that you have a non-combatant character does not mean you will get killed!
I hear it all the time from people making character cards, that they put a million-and-one crazy powers/abilities to their characters because they are afraid they are going to get killed.
There are dozens of characters out there that probably couldn't run away fast enough if someone decided to swing a sword at them, who have been alive and well since the beginning of the sim. Most notably, Fawn. Yeah she's been attacked before, but she's still alive and well!
Don't make a character with fear that it will get killed! Make a character you will enjoy playing.
If you have questions, please ask, and let's discuss them here to clarify things for everyone.