My kill player script isn't working?

players = game:GetService("Players")
admins = {Player = true}

players.PlayerAdded:connect(function(newPlayer)
    if admins[newPlayer.Name] then
        newPlayer.Chatted:connect(function(msg)
            if string.lower(msg):sub(1, 6) == ":kill " then
                for _, player in pairs (players:GetPlayers()) do
                    if player.Name:sub(1, #msg:sub(7)):match(msg:sub(7)) then
                        player.Character.Humanoid.Health = 0
                    end
                end
            end
        end)
    end
end)

Let's jump to lines 7 and 9 to see what they mean. Line 7:

if string.lower(msg):sub(1, 6) == ":kill " then

First, we have string.lower(msg). That means we're taking into account that the admin might be saying :Kill Pla or :KILL Pla. We then get the first 6 letters of that message: :kill. If that's the case, we move on.

if player.Name:sub(1, #msg:sub(7)):match(msg:sub(7)) then

We have msg:sub(7). That is, every character in msg from the 7th onward. Let's say that's "Pla". The length of "Pla" is 3. So, we want to see if the substring of a player's name from 1 to 3 is "Pla". If that's the case, we kill the dude.

Note that this will kill everyone who matches the shortened name. If you have 4 guests on your server, you can say :kill Guest to kill them all.

This should do it.

players = game:GetService("Players")
admins = {["Player"] = true ; }

players.PlayerAdded:connect(function(newPlayer)
if admins[newPlayer.Name] then
newPlayer.Chatted:connect(function(msg, speaker)
if string.sub(string.lower(msg),1,6) == ":kill" then
local player = findplayer(string.sub(msg,6),speaker)
if player ~= 0 then
for i = 1,#player do-
if player[i].Character ~= nil then
local human = player[i].Character:FindFirstChild("Humanoid")
if human ~= nil then
human.Health = 0
end end end end end

Hope it does the job for you.