Each week, the Fox thriller “Alcatraz” lets loose another former inmate of the island prison into modern-day San Francisco. And more than a few of those inmates, we’ve seen, have had some motivation for their criminal behavior.

Tonight’s episode of the series, “Johnny McKee,” offered the most overt explanation yet for what makes a killer a killer.

As Hauser (Sam Neill), Madsen (Sarah Jones) and Soto (Jorge Garcia) pursue McKee (Adam Rothenberg), a 1950s mass murder who killed with poison and is taking up his old habits in the modern-day, flashbacks show McKee as a man — admittedly unhinged and homicidal — bullied into killing another inmate while in prison.

There’s not a lot of sympathy to be had for McKee, of course. Ultimately he tells prison psychiatrist Lucy Banerjee (Parminder Nagra) — who also made the leap through time along with the inmates and Dr. Milton Beauregard (Leon Rippy) — the motivation for his first mass murder, more than a half-century ago. It’s pretty dire but doesn’t prompt viewers to think, “Yeah, I can totally see why he’s killing dozens of people.”

In the present day, Banerjee has been shot by a sniper and lies in a coma. Hauser, who knew Banerjee when he was a young guard, keeps careful — even loving — watch over the doctor.

The show, which has been struggling in the ratings, continues to tease with overall mythology and secrets. Madsen’s grandfather, an inmate on the loose in the present, is mentioned. There are also sinister overtones to the modern-day prison where Hauser — who we learn has the authority to eliminate viral videos from the Internet if they threaten to reveal the existence of his little project — keeps the recaptured inmates.

I’m still enjoying “Alcatraz,” but I’m increasingly worried that becoming involved in the show’s mythology — and that’s the best part of the show, really; the hunting down of inmates is becoming pretty routine — is going to pay off only in frustration when Fox yanks the show.

Next week’s episode, like an earlier one in which the first guard returned, looks to be interesting. An inmate who was innocent back in the day returns. But is he a killer now? (I’m guessing no.)