The king's justice, on the steps of Baelor's sept
Ned Stark works out the truth about Cersei's children, gives her the chance to run out of decency, and is betrayed for it. He confesses to treason to save his daughters ÔÇö and a boy king kills him anyway. The story announces here, unmistakably, what kind of story it intends to be.
The fuller tale
Ned Stark worked it out from a book: every Baratheon for three hundred years had been black of hair, and the king's three children were golden. What that meant was that Joffrey had no right to anything, that Stannis was the heir, and that the queen had been putting her brother's children in the line of succession for a decade.
Then he did the thing that killed him. He went to Cersei and told her he knew, and gave her the chance to take her children and run before he told Robert ÔÇö because he had seen what Robert did to Targaryen children and would not be part of it again. He gave his enemy warning out of decency, and she used it.
Robert died on a boar hunt with too much wine in him. Ned's claim to the regency rested on a piece of paper and the City Watch, and Petyr Baelish had bought the City Watch. He confessed to treason in front of the Great Sept ÔÇö for his daughters, who were in the castle ÔÇö and a boy king with an audience decided to be impressive.
The rules that did not apply
Every calculation Ned made was correct in the world he came from. He had the law, the truth, the rightful heir, the written testimony and the moral high ground, and he lost inside a week to a man with no name, a woman with no options, and a child with no restraint.
It is the moment the story tells the reader what kind of story it is. Nobody in this chronicle is safe because they are the honourable one; several people are in danger precisely because they are.
Elsewhere in the wiki
 See the world as it stood in 298 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.