I was faked out a bit by the "explosion" when the Doctor reset the Vardies - I assume it was meant to be the city (which was composed of them) momentarily disintegrating and then reassembling - the force of which knocked the humans out - but in the moment I thought the Doctor had triggered the fusion core and set the Vardies to default to their "save the humans" mode by literally rebuilding them - in the process gaining an understanding of death. Of course, this would also mean Bill and the Doctor were killed and rebuilt by the Vardies too, but that's hardly a concern for a guy who spent several million years dying and using the last of his life energy to power up a "respawn lol!" device. And, depending on how Rassilon gave Time Lords regeneration abilities in the first place, they may be integrated nanotech anyway.