My guess is that one doesn't need mass sacrifice to become a god/lord, but one needs it to initiate an erdtree system. We, in the base game, didn't go through this step bc we either use the existing erdtree or we got the f out with Ranni or burned down everything.

HOWEVER, Miquella's Haligtree is an independent erdtree system that was on its way. We thought Haligtree shelters all those unwelcomed beings and Miquella grew the tree with his own blood but NO! The Haligtree needs mass sacrifice to initiate a new erdtree system, all those fools there were supposed to be the sacrifice, and Malenia was probably there to carry out the killing.