In the original series, the Klingons were an aggressive military superpower with expansionist ambitions. In the Cold War politics of the time, they obviously represented the USSR, while the Romulans just as obviously represented Red China. They appeared little different from us; they could be violent, aggressive, sly, cloying, or deceptive, just like us. But at the end of "Errand of Mercy", Kor reflected wistfully upon the grand battle that never was: "it would have been glorious!" Oh, from such humble beginnings did such a vast mythology grow ...
But the TNG writers got it in their heads that Cold War politics were no longer appropriate in the politically correct 1990s, so they decided to rewrite the Klingons. What did they base the rewrite on? Kor's single line of dialogue. Kor mentioned his wistful desire for a "glorious" battle royale, and the ancient Vikings believed that death in battle was "glorious". That's enough of a connection for a brain bug; the writers decided that the Klingons had a similar history to our own, and that there were ancient Vikings in their past too. Naturally, the fans went along for the ride.
Time for this brain bug to start growing. In "Heart of Glory", Worf ran into Korris, a Klingon social reactionary who wanted to return to the bygone era of ancient warrior values. No big deal, right? This brain bug has grown as far as it's going to grow, right?
Wrong. Nobody seemed to notice that Korris was a dinosaur even among his own people. They noticed only that this was a cool new aspect of Klingon culture, so the writers grabbed this assumption and ran with it. They proceeded to construct an entire society around the notion that the Klingons were futuristic Vikings.
When they were space vikings with pointy teeth and anger management issues they were pretty cool but as with most armies the 5th drive for new minis and blowing open closed doors fucked them. All the non codex chapters suffered from it but the wolves became almost a parody of their former selves.