Select one random player? [ERROR]

local Players = game.Players:GetPlayers() --This returns a table containing all the players in the server
local RandomPlayer = Players[math.random(1,#Players)] --This picks a random player from that table
RandomPlayer.Character:BreakJoint() --This kills that random player

Hope this helped!

This problem does not require a loop. Since there's only one player you want to affect, there's no need to go over the whole thing (probably).

To select something randomly from a list (the list we will be using is GetPlayers) we have to generate a random index from the list (1, 2, 3, 4, ... , length of list - 1, length of list). We do that with this simple line:

local randomIndex = math.random(1 , #list);

Then, to get the random object from the list, we just have to ask for the object at that index:

local randomObject = list[randomIndex];

For getting a random player, it would look like this:

local allPlayers = game:GetService("Players"):GetPlayers();
if #allPlayers >= 1 then
    -- Selecting a random index from a length 0 list
    -- will result in an error, so we can only do
    -- something if the list has at least one thing
    local randomPlayerIndex = math.random(1,#allPlayers);
    local randomPlayer = allPlayers[randomPlayerIndex];
    if randomPlayer.Character then
        randomPlayer.Character:BreakJoints();
    end
end