How do I end a match of something after all players but one are dead?

Quite easily, really.

One way you could accomplish this is by adding all players to a table upon the match beginning, as well as connecting a Died event with each of their Humanoids. When one of them dies, you remove them from the table (or otherwise tag them). When the table has nobody left, you end the match. If the table only has one person remaining, you end the match with your winner.

Below is an example of how you would add every player to a table, as well as remove them from the table upon their death.

local InGame = {}

function Find(Item, Table, Remove)
    for i, v in pairs(Table) do
        if v == Item then
            return Remove and table.remove(Table, i) or Item 
        end
    end
end

for _, Player in pairs(game.Players:GetPlayers()) do
    if Player.Character and Player.Character:FindFirstChild("Humanoid") then
        InGame[#InGame +1] = Player
        Player.Character.Humanoid.Died:connect(function()
            Find(Player, InGame, true) 
        end)
    end
end