On Titan, Thanos watched an overcrowded world collapse under its own weight, and drew a conclusion no one else was willing to share with him: the universe doesn't die of cruelty, but of neglect.
Since then, every move he makes follows one logic, cold and almost tender in his own eyes, to give death what she has quietly demanded all along, a balance that life, left to itself, never manages to impose on its own.
The priests of Sithis would likely call him a believer without knowing it, an outsider who understood before they did that the void isn't an absence to fill, but an order to restore.
Sithis asks for no sacrifice, no prayer, only that the world return to the place it should never have left, and that is precisely what Thanos came searching for among the realms of Tamriel: somewhere his philosophy, condemned everywhere else, finally finds an echo that doesn't try to argue back.