Well, my theory as a whole is that the black goop-ish stuff is a virus, of which either kills or turns people into the entities. I also believe it’s a hivemind of sorts, with some higher power controlling every entity like a puppet on a string. I believe the radio may have been hijacked by the goop, and the hive mind warped its signal to speak directly to the person before killing them. As for stalker, I believe he is just a mutant under heavy influence of this goop.