Milton, Hester van Zandt's right-hand man, is a two-faced sneak. He eavesdrops on Luisa's conversation with Hester and then calls Joe Napier to tell him that Luisa is still alive—in exchange for a sweet wad of cash, naturally.
So, Milton's the type of guy who masquerades as a do-gooder—he's one of the protestors of the Swannekke reactor—but is actually selling the real do-gooders' secrets to the people they're fighting against. That just might make him lower than the guys who are behind this whole conspiracy. Well, except the murderers. At least Milton doesn't kill anyone.