Wont add anything to the leaderboard?

local Humanoid = script.Parent.Zombie -- Or Zombie Or Whatever 
function PwntX_X() 
local tag = Humanoid:findFirstChild("creator") 
    if tag ~= nil then 
        if tag.Value ~= nil then 
local Leaderstats = tag.Value:findFirstChild("leaderstats") --Im pretty sure your problem is here, it is looking for "leaderstats" inside of a value?
            if Leaderstats ~= nil then 
Leaderstats.Heads.Value = Leaderstats.Heads.Value + 8
wait(0.1)
script:remove()
            end 
        end 
    end 
end 
Humanoid.Died:connect(PwntX_X) 

Try this

local Humanoid = script.Parent.Zombie -- Or Zombie Or Whatever 
function PwntX_X() 
local tag = Humanoid:findFirstChild("creator") 
    if tag ~= nil then 
        if tag.Value ~= nil then 
local Leaderstats = game.Players:findFirstChild(tag).leaderstats 
            if Leaderstats ~= nil then 
Leaderstats.Heads.Value = Leaderstats.Heads.Value + 8
wait(0.1)
script:remove()
            end 
        end 
    end 
end 
Humanoid.Died:connect(PwntX_X) 

I might not be spot on with my answer, but gives this a shot