Billy is encountered the first time you enter the Pitt steelyard. Should the player chose not to kill him on their first encounter, Billy is never seen again (unless on subsequent visits the player chooses to assume that one of the many generic trogs in the steelyard is Billy.)
His brother John-John can be seen talking to him, trying to tap any human reason still left in Billy. It is soon revealed that Billy, however, has lost all of his ability to reason, and soon the one-sided conversation deteriorates with John-John running to a locked gate screaming for help. At this point, Billy attacks John-John. John-John will not fight back, and should the player not intervene, Billy will kill him. He will then end his scripted events and, like any other trog, attack the player.
Should the player make a successful sneak attack on Billy, but fail to kill him, he may immediately attack and kill John-John before seeking out the player.
Should the player kill Billy and save his brother, John-John will walk over to Billy's corpse and cry. He offers no thanks or dialog options; when conversation is attempted, he only pouts a pitiful "Billy..." and continues to sob.