Script checks if player is on red team then gives the player one point (check more text . -?)

Instead of using the StarterGui to make the frame visible, you should use the PlayerGui inside the Player, since everything inside StarterGui will replicate inside Player.PlayerGui when the player joins. Another reason is that if you tried to edit it in StarterGui, it won't affect PlayerGui, and StarterGui is only accessible in the client (LocalScripts).

To kill every player in the server, you can get an array of all players in the server using Players:GetPlayers() and iterate through the array using either pairs() or ipairs(). ipairs() is recommended for arrays because it is faster. To kill the player, you can use either Player:LoadCharacter() or Player.Character:FindFirstChildOfClass("Humanoid").Health = 0. The difference between the two is their RespawnTime. Player:LoadCharacter() respawns the player immediately, while Player.Character:FindFirstChildOfClass("Humanoid").Health = 0 respawns after 3 seconds (default).

Final Script:

local Players = game:GetService("Players")
local Teams = game:GetService("Teams")

local function killEveryPlayer()
    for _, player in ipairs(Players:GetPlayers()) do
        task.spawn(function()
            local character = player.Character
            if (not character) or (not character.Parent) then
                character = player.CharacterAdded:Wait()
            end
            character:FindFirstChildOfClass("Humanoid").Health = 0
        end)
    end
end

local function redTeamScoredFrame(visible: boolean)
    for _, player in ipairs(Players:GetPlayers()) do
        task.spawn(function()
            player:WaitForChild("PlayerGui"):WaitForChild("RedTeamScored"):WaitForChild("Frame").Visible = visible
            player:WaitForChild("PlayerGui"):WaitForChild("DeathGui").Enabled = not visible
        end)
    end
end

local debounce = false
script.Parent.Touched:Connect(function(hit)
    local player = Players:GetPlayerFromCharacter(hit:FindFirstAncestorOfClass("Model"))
    if (player and player.Team.Name == "Red Team") and (debounce == false) then
        debounce = true
        player.leaderstats.Points.Value += 1

        task.spawn(killEveryPlayer)
        task.spawn(redTeamScoredFrame, true) -- makes RedTeamScore Frame visible

        task.wait(3)
        task.spawn(redTeamScoredFrame, false) -- makes RedTeamScore Frame invisible
        debounce = false
    end
end)

to T3_MasterGamer's comment:

This first script is in the DeathGui:

local cam = workspace.CurrentCamera
local plr = game.Players.LocalPlayer or game.Players.PlayerAdded:Wait()
local char = plr.Character or plr.CharacterAdded:Wait()
local hum = char:WaitForChild("Humanoid")
local imalabel = script.Parent.Frame.ImageLabel
local texlabel = script.Parent.Frame.TextLabel

hum.Died:Connect(function(killed)
    local killer = char.Humanoid:FindFirstChild("creator")
    if killer.Value then

        local killerChar = killer.Value.Character
        local killerName = killer.Value.Name
        local killerId = killer.Value.UserId
        local image, isReady = game.Players:GetUserThumbnailAsync(killerId, Enum.ThumbnailType.HeadShot, Enum.ThumbnailSize.Size180x180)

        cam.CameraSubject = killerChar.Humanoid
        script.Parent.Frame.Visible = true
        imalabel.Image = image
        texlabel.Text = killerName
        wait(10)
        cam.CameraSubject = hum
        script.Parent.Frame.Visible = false

    end
end)

This script is in ServerScriptService:

local players = game.Players
local t = 5

players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(character)
        local humanoid = character:WaitForChild("Humanoid")

        humanoid.Died:Connect(function(died)
            player.PlayerGui:FindFirstChild("DeathGui"):WaitForChild("Frame").Visible = true

            for i = t,0,-1 do
                player.PlayerGui:FindFirstChild("DeathGui").Frame.Timer.Text = "Respawning in"..i
                wait(1)
            end
            player.PlayerGui:FindFirstChild("DeathGui"):WaitForChild("Frame").Visible = false
        end)
    end)
end)