Ray Seward was hanged on Sunday. By the neck until dead, as the 19th century phrase might have had it.

And that, both tragically and gruesomely, took a while which made Sunday’s episode of “The Killing” on AMC as powerful a rendering of capital punishment as I’ve ever seen on TV.

Certainly, Sunday’s episode of “The Killing” justified the entire three seasons of AMC’s TV noir, thus far, which have ridden a roller coaster of public affections since that first season made a big deal out of telling us how much we’d care “who killed Rosie Larsen” and then ended the season without telling us.

At that moment, every faithful watcher of the show wanted to hang show-runner Veena Sud off a cliff, along with a few of the other writers and directors, too.

I must say, it was nice of them all to answer their own question by the end of season two. By the time we found out who killed Rosie Larsen, it turned out to be so convoluted and strung out that it was virtually impossible to care, even though the identity of the actual killer should have hit us right between the eyes.

With all of that unintentional audience bashing going on, almost anyone could be forgiven for wondering why the canny folks at AMC – who, after all, cart away Emmys for “Mad Men” and “Breaking Bad” in a truck – kept on renewing the show.

Easy. It’s extraordinary. It was from the beginning and has never stopped being so. What we’re watching in perennially dark, rainy Seattle is the immensely compelling relationship of two deeply damaged and brilliant homicide cops: one a pint-size obsessive (played by Mireille Enos) whose attention to her investigations virtually jettisoned (by season three) all possibilities for private fulfillment; the other a tall, loosey-goosey, substance-abusing intuitive (Joel Kinnaman) who has a way with troubled kids and a tendency to fall off the wagon when things get too intense.

When everything else about “The Killing” was either going off the rails or in imminent danger of doing so, you couldn’t take your eyes off these two depressives, whose wounded psyches seemed somehow to mesh perfectly.

A lot depends on the individual charisma of the two actors – Enos as Linden and Kinnaman as Holder. But so, too, did an enormous amount depend on the acting style encouraged by AMC’s shows, in which matter of fact sotto voce volume levels are encouraged whenever nothing overtly dramatic is going on. The result was, not surprisingly, more dramatic intensity in the subtle communion between the two cops than we are used to seeing almost anywhere.

Despite the incongruity of their physical differences (tiny; tall), there was, in the way they talked to each other, a sense of genuine partnership that most other TV shows have to convey through writing. On “The Killing,” you could see it and hear it on their faces and in their perfectly modulated tones of voice.

It was stunning – a portrait of occupational depressives whose grip on life seemed codependent (as they like to say in recovery and psychology circles).

The new season tried to convince us that Holder and Linden had split before the season and come back together to re-investigate a Linden case that was about to send Ray Seward to death by hanging. It tried to sell us on a relationship Holder had with a corrupt cop partner (played by the always suspicious Gregg Henry) that was all writerly invention, contradicting the evidence of our own two eyes whenever the show’s two most important actors are on screen together.

You could call it “sexual tension” except that it’s the show’s power that what we’re watching so clearly goes so much deeper than sex. These two physical opposites seem to run on the same tracks. They “get” each other in ways most of the human race doesn’t.

When, in the third-to-last episode, Holder finally went in for a kiss, Linden showed the tiniest tremor of temptation and then smiled and backed away in a move both instantly understood.

A serial killing case presented itself in this season and it will all, no doubt, be resolved in the two-hour final episode at 9 p.m. Sunday. They’d never have the temerity to again pull a stunt like the inconclusive finale of Season One.

But the finale of this season was the incredible episode on Sunday which, for an hour of prime time TV, gave us a portrait of capital punishment that almost immediately transcended cliché and became a harrowing hour of genuinely classic television.

The cliché was in the tale of an eleventh-hour attempt to save a condemned prisoner from execution. Will the attorney general grant a stay, etc.? But the power lay in the details of the state’s sanctioned murder of convicted killer Ray Seward, whom Linden was almost certain was innocent of his wife’s murder.

Peter Sarsgaard played Seward, and it’s the finest work I’ve ever seen him do. We watched as a condemned man fell apart in pieces in front of us – unable to say goodbye to his own son or to walk unaided to the gallows.

We watched the alternating bullying and tenderness of the prison guards forced to put him on the gallows. (What I will never forget from the brilliant direction of Nicole Kassell is the close-up of the condemned man’s calves as the guard tenderly tapped one of them, so that the prisoner would squeeze them together and allow metal cuffs to be placed around his ankles.)

And then, for the episode’s harrowing and classic TV finale, we were confronted with what had been the condemned man’s greatest fear all along, a hanging gone wrong. From sound alone, we knew that death wasn’t instantaneous from the snap of a neck but would require strangling and the cessation of breathing.

And, most horrifying of all, we were left to wonder if the hostile chief prison guard’s (Hugh Dillon) inability to place the final hood over Seward’s head weren’t, in fact, an indication of guilt over hideously rigging the execution for exactly that final result.

A truly astonishing hour of prime time television – singular and, in every way, head and shoulders above everything else on “The Killing” itself and AMC in general.

Sunday’s two-hour finale will be an anticlimax.

It’s almost as if show-runner Sud were showing us, not telling us, on Sunday how truly little import there is to a mere wrapping up of a murder plot.