They - change of person from “ye” (Mic 3:9); the third person puts them to a greater distance as estranged from Him. It is, literally, “Whosoever builds,” singular.
build up Zion with blood - build on it stately mansions with wealth obtained by the condemnation and murder of the innocent (Jer 22:13; Eze 22:27; Hab 2:12).