In a Would You Believe special on RTÉ1, reporter Mick Peelo pondered the allure of evil with various psychologists and criminologists, but he could have saved himself the bother simply by watching Mr Mercedes on the same channel.
Indeed, one of the unsettling master strokes of this drama, which ended on Tuesday night, was that it made you complicit with the dreadful doings of young psychopath Brady Hartsfield, whose villainy you knew from the outset and in whose creepy company you spent every second scene for 10 whole weeks.
This was a trick that Patricia Highsmith had also pulled off in her Tom Ripley novels, where readers, against their better judgment, were so fascinated by the nonchalantly murderous anti-hero that they kept hoping he wouldn't get caught, but I've never seen it managed so compellingly on screen before.
This was largely due to Harry Treadaway's mesmerising performance, so nuanced and detailed that you felt you knew Brady even more intimately than you knew crotchety retired cop Bill Hodges, whose life he was tormenting and who was splendidly played, without any attempt at ingratiation, by Brendan Gleeson.
But the genius of this series was that all of the characters, even the most minor, were so quirky and so well cast that they seemed to have lives that extended beyond their presence on screen - so that, by the end of this week's final episode, you wondered how those of them who had survived Brady's murderousness would fare in their future lives.
Not many series leave you feeling that, but then Mr Mercedes was exceptional - the year's best drama by a long shot - so all credit to RTÉ for acquiring it even before it's had a network airing in its native America.
But I still don't know quite what to make of the Alison Spittle sitcom, Nowhere Fast (RTÉ2) - or, rather, I still can't get a fix on the central character she plays. Angela's meant to be a savvy young media figure whose career-ending broadcasting gaffe has sent her scurrying home to the midlands, but most of the time she comes across as someone so clueless she should be in a home for the bewildered.
Maybe I'm missing something, but in this week's second episode it was Clare Monnelly's splendidly vituperative turn as feckless sidekick Mary that again registered most strongly. Some of the gags and repartee were good, too, while there was a real sense of rural small-town life, with all its petty social constraints and resentments.
I just wish that the show was either funnier or darker, though both would be even better, as in Phoebe Waller-Bridge's Fleabag or Stefanie Preissner's Can't Cope, Won't Cope, whose forthcoming second season should be welcomed.
There wasn't much to be welcomed in the rest of this week's offerings, the aforementioned Would You Believe (RTÉ1) making heavy weather of its ruminations about evil.
"Evil, what is it?" Mick Peelo asked at the outset, while at the very end he wondered "Are we any the wiser now?" Well, I certainly wasn't, even though he'd gone to strenuous lengths to tease out the matter.
We heard from exorcists, folklorists, psychotherapists, neuroscientists and criminologists, all of them giving their professional tuppence worth, and we heard, too, from the parents of Rachel O'Reilly, who had been murdered in her north Dublin home by her husband in 2004, and yet answers to the reporter's initial query weren't forthcoming. Perhaps they'll be provided in tomorrow night's concluding instalment of this two-parter. But if evil remains a troubling enigma, romance in all its heady mystery was being celebrated in Elizabeth and Philip: Love and Duty (BBC1).
This tribute to the 70-year marriage of the British monarch was presented by Kirsty Young and within five minutes I felt as if I were drowning in treacle.
Philip was a "dashing young naval officer" and their "fairytale wedding" not only "captured the imagination of a nation" but led to "one of the greatest and most enduring love stories of our time". And to show that, in many respects, the couple were just like anyone else, we got to meet other couples who had also married 70 years ago, even though none of them lived in palaces or had their lifestyles funded by the taxpayer.
We also got to meet Gyles Brandreth, who has spent his life sucking up to royalty, and it was at this point I decided that life was too short for any more of this toadying twaddle.
Instead I watched this week's episode of Blue Planet II (BBC1) while reflecting that if Trump and his climate-denying cronies have their way, the wonders of the deep won't last an awful lot longer. But what wonders this series is showing us, from spinner dolphins searching for food in shoals of 5,000 to lantern fish being pursued by roving gangs of yellow-fin tuna. It's a predatory world down there but astonishing to behold. May David Attenborough live forever.
In Gregory Porter's Popular Voices (BBC4), a three-part documentary series that ends next Friday, the amiable American singer celebrates those vocalists whose "sublime technique and power to move us to tears" have made them famous.
His choices are eclectic and I wouldn't agree with some of them (Whitney Houston and Mariah Carey preferred over Peggy Lee and Sarah Vaughan), but he was very good on Mahalia Jackson, Ella Fitzgerald and Aretha Franklin, and he was interesting on Freddie Mercury, whom he described as "an opera singer with a rock band", even if I'd opt for Mercury's own take on himself as "a Persian popinjay".
And Porter's love for his subject was never in doubt.