Arduino in-built LED is not turning off
If you don't tell it to turn off, it behaves "as it wants". If you don't write a 0 on its pin, how can you force it to stay shut down?

If you want it to be in an off state, force it: in your setup function write

pinMode(13, OUTPUT);
digitalWrite(13, LOW);


No need to refresh it every loop, just in the setup.