Simple KO script not working?

Try this:

function tagHumanoid(humanoid)
 local creator = Instance.new("StringValue") 
local plrName = script.Parent.Parent.Parent.Parent.Name
local plr = game.Players:FindFirstChild(plrName)
 creator.Name = "creator" 
creator.Value = plr
creator.Parent = humanoid 
end

My changes: Line2: local creator = Instance.new("StringValue") I removed the "B", because it was parenting to B.

Inserted lines 3-4 local plrName = script.Parent.Parent.Parent.Parent.Name local plr = game.Players:FindFirstChild(plrName) Just a simple fix. I personally do not know if it actually makes a difference, but it looks a lot cleaner.

function tagHumanoid(humanoid) 
    local creator = Instance.new("StringValue",b) -- b is the name of the bullet 
    creator.Name = "creator" 
    creator.Value = game.Players[script.Parent.Parent.Parent.Parent] 
    creator.Parent = humanoid 
end

First of all, don't put all the code on one line like that. Not sure if you did that on accident, but it breaks the code..

Secondary, you are trying (Not successfully doing so) to set the value of a StringValue to a Player Object. This is not possible, the value has to be a string. You either have to use an object value, or set the value to the name of player.

Using an object value:

function tagHumanoid(humanoid) 
    local creator = Instance.new("ObjectValue") -- b is the name of the bullet 
    creator.Name = "creator" 
    creator.Value = game.Players[script.Parent.Parent.Parent.Parent] 
    creator.Parent = humanoid 
end

Using a string as value:

function tagHumanoid(humanoid) 
    local creator = Instance.new("StringValue") -- b is the name of the bullet 
    creator.Name = "creator" 
    creator.Value = script.Parent.Parent.Parent.Parent.Name
    creator.Parent = humanoid 
end

Btw, setting the parent of the value to b seems to be unnecessary, as it is parented somewhere else later on. You can change properties of objects even if they are not parented. Not having access to the full scripts I cannot tell if anything depends on these being parented to b, so you can always change it back in case something breaks.

Updated full code paste: http://pastebin.com/eMkUEqrQ

Sorry for the bump