How Do I know when a player stopped walking?

I want to make a script that a player dies after he stopped walking. But I can't find the right Property to find it? Can someone help me??

Here's how my finished script would look like:

--LOCAL SCRIPT

player = game.Players.LocalPlayer
Person = player.Character
DontStop = false

game.Workspace.Triggerpart.Touched:connect(function(hit)
if hit.Parent.Name == player.Name then
Person.Humanoid.Walkspeed = 100
DontStop = true
end

--Help here: When the player stops walking and DontStop == true then...
Person.Humanoid.Health = 0

Your help will greatly be appreciated. If you wish, your username may be posted under the Minor Credits section in my game.

Another thing you could do would be getting when an idle animation plays, they die. Look at the custom animations article on the wiki.