Why does this simple kill script not work?

This will work, but only as long as the touched never errors.

A lot of contacts, though, would cause this to error.

Always include robust error checking in your code.

script.Parent.Touched:connect(function(hit)
    print(hit)
    if hit.Parent and hit.Parent:FindFirstChild("Humanoid") then
        hit.Parent.Humanoid.Health = 0
    end
end)