can someone help me script it so that if a player kills themself they get kills subtracted?

To put a cap to the player's kills so it doesn't go lower than 0. You have to make a PropertyChangedSignal(), This makes it so whenever the value of your kills change. It will fire a function.

So what we do with this function is:

kills:GetPropertyChangedSignal("Value"):Connect(function() --If value changes, It fires a function.

    if kills.Value < 0 then --If the kills value is under 0, It fires the code underneath
        kills.Value = 0 --Changes kills value to 0
    end

end)

I have no idea how the murderer value works, but you'd need to create a value of Who the murderer is and check if the Murderer killed the player. If not, You remove 1 kill from the player.

So something like this:

    local MurderFolder = Instance.new("Folder") --Makes MurderFolder (To keep things neat)
    MurderFolder.Name = "MurderFolder"
    MurderFolder.Parent = player

    local DmgMurderer = Instance.new("BoolValue") --Makes Boolvalue, Checking if player got killed by Murderer.
    DmgMurderer.Name = "KilledByMurderer"
    DmgMurderer.Parent = MurderFolder

    player.CharacterAdded:Connect(function(char)

        char:WaitForChild("Humanoid").Died:Connect(function() --If player died, Fires Function.
            if DmgMurderer.Value ~= true then --If the murderer hasn't killed the player, fires code underneath.
                kills.Value = kills.Value - 1 --Removes a kill.
            end
        end)

    end)

How to make this work? You can find out if the player has been hit by the murderer's weapon and check if their health is 0, If so, you change DmgMurderer.Value to true.

This is all I thought of but yeah, hopefully this helps somehow.

This answer is quite simple: compare the murderer (ObjectValue) to the player:

player.CharacterAdded:Connect(function(char)

    char:WaitForChild("Humanoid").Died:Connect(function()
        local creator = char:WaitForChild("Humanoid"):FindFirstChild("creator")

        if creator and creator.ClassName == "ObjectValue" then
            local murderer = creator.Value

            if murderer == player then
                murderer:WaitForChild("leaderstats"):WaitForChild("Kills").Value -= 1 -- they killed themself
            else
                murderer:WaitForChild("leaderstats"):WaitForChild("Kills").Value += 1
            end

        end
    end)

    if kills.Value >= 10 then

        char:FindFirstChild("hat"):Destroy()

        game.Lighting.hat1:Clone().Parent = char

    end
end)