Detect player in area and kill code doesnt work, How can I fix this?

There are a few issues with what you're doing. Let me break it down for you.

Part.Touched:Connect(function(object)
    if object.Parent:FindFirstChild("UpperTorso") then
        local player = object.Parent:FindFirstChild("UpperTorso").Parent
        local localplayer = game.Players:FindFirstChild(player.Name)
        local playerteam = localplayer.Team
        if playerteam.Name == "Axis" then
            print("Detect")
            wait(3)

With this, I'm going to assume this is an R15 game only, In which using UpperTorso is okay. In my demonstration, I used HumanoidRootPart in which is valid for both R6 and R15.

also, I used

game:GetService("Players")

for the reason of game. Players could come up nil if the Players Service name was changed (Yes this is possible lol)

Another issue here is you are taking when they get detected and just waiting 3 seconds. What happens if they leave after 1.3 seconds? It Is still going to continue waiting that last 1.7 seconds.

Also, the reason why it continues to explode even if you are not in the area is that there is nothing stopping the function from firing again with each individual part on the player's character. (Left hand, left lower arm, left upper arm and so on) Basically, if the part is big enough this function will fire 16+ times ( R15 + HumanoidRootPart and then some due to touched being sensitive) Example -- this is just from spawning in and falling into the part

So what you are going to want to do is add something to prevent that, as I did in my example was just checking to see if the object that touched was named HumanoidRootPart after it checked to see if it was within the character.

That's about it. Here's my completed code for this. Please do yourself a favor and take this code to learn from. Little advice, don't just copy and paste it. Have it side by side of what you're doing and rewrite the entire thing. (It's only like 70 lines, lol you'll be fine)

local timeToDetonate = 3

local Players = game:GetService("Players")

local part = script.Parent

local alreadyWaiting = {}

local function detionatePart(character)
    local explosion = Instance.new("Explosion")
    explosion.ExplosionType = Enum.ExplosionType.NoCraters
    explosion.Position = character.UpperTorso.Position
    --explosion.DestroyJointRadiusPercent = 0
    explosion.BlastPressure = 1
    explosion.Parent = game:GetService("Workspace")

end

local function startTimer(player,detonationTime)
    print("Been Detected")
    local begin = tick()
    local timePassed = 0
    repeat
        timePassed = tick() - begin
        wait()
    until timePassed >= detonationTime or alreadyWaiting[player] == nil
    return timePassed >= detonationTime and true or false
end

local function resetWaiting(player)
    if player then
        if alreadyWaiting[player] then
            alreadyWaiting[player] = nil
        end
    end
end

part.Touched:Connect(function(object)
    if object.Name ~= "HumanoidRootPart" then return end
    local player = Players:GetPlayerFromCharacter(object.Parent)
    if player then
        local character = object.Parent
        local humanoid = character:FindFirstChildOfClass("Humanoid")
        if humanoid == nil or humanoid.Health <=0 then return end
        if player.Team.Name == "Axis" then
            if alreadyWaiting[player] ~= nil then return end
            alreadyWaiting[player] = true
            local detonate = startTimer(player,timeToDetonate)
            if detonate then
                detionatePart(character)
                resetWaiting(player)
            else
                print("you are safe")
            end
        end
    end
end)

part.TouchEnded:Connect(function(object)
    if object.Name ~= "HumanoidRootPart" then return end
    local player = Players:GetPlayerFromCharacter(object.Parent)
    resetWaiting(player)
end)

Players.PlayerRemoving:Connect(function(player)
    resetWaiting(player)
end)

If you have any questions, PLEASE feel free to ask. Even feel free to message me on Roblox! It.

Also, if you check here, I have an Example Place That explains everything down to what "end" does. That place is where I took your code and wrote my own and even tested things.