Every Ninja will automatically become a playerkiller as soon as they finish the Ru Noperi at guild level 175. You may of course choose to become a playerkiller before then, but it is not required.
Quite simply, because our guild is not based on the ninja of Japan, but on the assassins from Interesting Times. Here is the official explanation from Kaylar:
Kaylar wisped: The long-winded explanation: originally you weren't "ninjas", you were "assassins". Kaylar wisped: And you were based more on the assassins in Interesting Times. Kaylar wisped: They used poisons, and blowpipes were the only things that were around that used poison at the time. Kaylar wisped: Then someone suggested that you should be called ninjas, and I agreed, not knowing all the expectations attached. Kaylar wisped: Apparently, ninjas should throw weapons, use faith, kill twenty foes at a time with evil death looks and fly. Kaylar wisped: And not be mutant turtles :(
At guild level 50 you will take the Chunin test with Master Yung to gain the inhume command and continue advancing. At this point you may only view and close NPC contracts on the scroll in the guild. At guild level 175 - and no sooner - you may attempt the Ru Noperi. Once the Ru Noperi has been successfully completed you will become PK and may view and close PK contracts on the scroll (if you have already closed at least four NPC contracts, that is).
To pass the Chunin test and obtain the inhume command you will have to find the contractable NPCs and find out information about them. This is an important part of your training because it encourages you to explore and find out where the various NPC contracts can usually be found (most don't move around much!). If you are having trouble with this part of your training most Ninja are willing to offer some advice, but please don't expect much help unless you're willing to make some effort to find these NPCs yourself.
There is no need to officially accept a contract. If a contract is listed on the scroll you need simply to find the target and attempt to inhume it. If you are successful, payment will be waiting for you at the guild.
There is a hidden room in the guild where you can find feather dusters, shovels, and other tools for completing your chores as a Genin or Chunin. Inspect everything in the guild thoroughly to find the room, or ask Master Yung and he may help you.
Weapons are banned in Agatea, so there are few places you can buy a weapon in the city. Upstairs in the dormotory there are usually a few weapons in the communal chest. There is a hidden shop in the guild where you can buy weapons. To find it, look closely at everything you find in the guild. If all else fails, ask an older Ninja for help and they will most likely be able to get you a weapon.
Weapons are banned in Agatea, so you must have the proper paperwork to carry a weapon on the streets. If an Imperial Guard catches you with a weapon out they will confiscate it and you will have to buy it back. You can buy permits from the shifty looking men on Tuna Walk or the dock near Long Tang Lane. Beware, your permit will expire, so keep an eye on the date.
There are four separate assassin guilds on Discworld, the Ninja, Ankh-Morpork Assassins, Mano Rossa, and Hashishim. Though each of these guilds provides a similar service, they each have their own guild house and player administration. Under normal circumstances the four guilds work together peacefully, allowing members of the other guilds to use their training facilities and close their contracts. It's important to note that the AM Assassins have a number of outposts in town besides Ankh-Morpork, such as Sto-Lat and Ohulan Cutash. The Ninja guild is not one of these outposts.