Why writing changes . . . I could almost write a novel, but the Klingon Warrior race was implied in TOS, at least once. It's true that mostly they were just "the enemy" and there was little examination into their society or beliefs, but in the episode Friday's Child, some of the Klingon views were made clear.
Kras, the Klingon, said to Teer, the leader of the tribes:
What do Earth men offer you? What have you obtained from them in the
past? Powders and liquids for the sick? We Klingons believe as you do.
The sick should die. Only the strong should live.
In the movies as well, the Klingon Warrior race was explored prior to TNG. Star Trek III, which came out 3 years before TNG featured the new Klingon Look and indicated some warrior aspects to their society.
Not wanting to re-watch the movie, I remember off hand that Christopher Lloyd played the Klingon Captain with a distinctly combative personality, having a ferocious pet that his subordinates were afraid to feed as one example and I'm vaguely remembering him choking a subordinate as well, so in both TOS and in the 3rd film there were indications in that direction.
It's also clear, in TNG that the Klingons aren't always honorable. They say they are, and they're warriors, but they lie, there's a point where one tries (unsuccessfully) to bribe data, they conspire. There's an episode where the Klingon leader is being poisoned and he asks Picard to investigate and find out who's doing it.
The big change is that TNG goes into the Klingon society in far greater detail, but I don't see the inconsistency that you see. There's indications that they're a warrior race prior to TNG and there's evidence that they are not always honorable during TNG.